これは、エキスパンダーのデフォルトテンプレート(ToggleButton ContentPresenter)が原因です。これは変更されたテンプレートであり、これが必要な結果であることを確認してください
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Class="WpfApplication19.MainWindow"
x:Name="Window"
Title="MainWindow"
Width="640" Height="480">
<Window.Resources>
<Style x:Key="ExpanderRightHeaderStyle" TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Padding="{TemplateBinding Padding}">
<Grid SnapsToDevicePixels="False" Background="Transparent">
<Grid.RowDefinitions>
<RowDefinition Height="19"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid>
<Grid.LayoutTransform>
<TransformGroup>
<TransformGroup.Children>
<TransformCollection>
<RotateTransform Angle="-90"/>
</TransformCollection>
</TransformGroup.Children>
</TransformGroup>
</Grid.LayoutTransform>
<Ellipse x:Name="circle" Stroke="DarkGray" HorizontalAlignment="Center" VerticalAlignment="Center" Width="19" Height="19"/>
<Path x:Name="arrow" Stroke="#666" StrokeThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center" SnapsToDevicePixels="false" Data="M 1,1.5 L 4.5,5 L 8,1.5"/>
</Grid>
<ContentPresenter HorizontalAlignment="Center" Margin="0,4,0,0" VerticalAlignment="Top" SnapsToDevicePixels="True" Grid.Row="1" RecognizesAccessKey="True"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Data" TargetName="arrow" Value="M 1,4.5 L 4.5,1 L 8,4.5"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF3C7FB1"/>
<Setter Property="Stroke" TargetName="arrow" Value="#222"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF526C7B"/>
<Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
<Setter Property="Stroke" TargetName="arrow" Value="#FF003366"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderUpHeaderStyle" TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Padding="{TemplateBinding Padding}">
<Grid SnapsToDevicePixels="False" Background="Transparent">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="19"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid>
<Grid.LayoutTransform>
<TransformGroup>
<TransformGroup.Children>
<TransformCollection>
<RotateTransform Angle="180"/>
</TransformCollection>
</TransformGroup.Children>
</TransformGroup>
</Grid.LayoutTransform>
<Ellipse x:Name="circle" Stroke="DarkGray" HorizontalAlignment="Center" VerticalAlignment="Center" Width="19" Height="19"/>
<Path x:Name="arrow" Stroke="#666" StrokeThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center" SnapsToDevicePixels="false" Data="M 1,1.5 L 4.5,5 L 8,1.5"/>
</Grid>
<ContentPresenter HorizontalAlignment="Left" Margin="4,0,0,0" VerticalAlignment="Center" SnapsToDevicePixels="True" Grid.Column="1" RecognizesAccessKey="True"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Data" TargetName="arrow" Value="M 1,4.5 L 4.5,1 L 8,4.5"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF3C7FB1"/>
<Setter Property="Stroke" TargetName="arrow" Value="#222"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF526C7B"/>
<Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
<Setter Property="Stroke" TargetName="arrow" Value="#FF003366"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderLeftHeaderStyle" TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Padding="{TemplateBinding Padding}">
<Grid SnapsToDevicePixels="False" Background="Transparent">
<Grid.RowDefinitions>
<RowDefinition Height="19"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid>
<Grid.LayoutTransform>
<TransformGroup>
<TransformGroup.Children>
<TransformCollection>
<RotateTransform Angle="90"/>
</TransformCollection>
</TransformGroup.Children>
</TransformGroup>
</Grid.LayoutTransform>
<Ellipse x:Name="circle" Stroke="DarkGray" HorizontalAlignment="Center" VerticalAlignment="Center" Width="19" Height="19"/>
<Path x:Name="arrow" Stroke="#666" StrokeThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center" SnapsToDevicePixels="false" Data="M 1,1.5 L 4.5,5 L 8,1.5"/>
</Grid>
<ContentPresenter HorizontalAlignment="Center" Margin="0,4,0,0" VerticalAlignment="Top" SnapsToDevicePixels="True" Grid.Row="1" RecognizesAccessKey="True"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Data" TargetName="arrow" Value="M 1,4.5 L 4.5,1 L 8,4.5"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF3C7FB1"/>
<Setter Property="Stroke" TargetName="arrow" Value="#222"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF526C7B"/>
<Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
<Setter Property="Stroke" TargetName="arrow" Value="#FF003366"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderHeaderFocusVisual">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate>
<Border>
<Rectangle Stroke="Black" StrokeDashArray="1 2" StrokeThickness="1" Margin="0" SnapsToDevicePixels="true"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderDownHeaderStyle" TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Padding="{TemplateBinding Padding}">
<Grid SnapsToDevicePixels="False" Background="Transparent">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="19"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Ellipse x:Name="circle" Stroke="DarkGray" HorizontalAlignment="Center" VerticalAlignment="Center" Width="19" Height="19"/>
<Path x:Name="arrow" Stroke="#666" StrokeThickness="2" HorizontalAlignment="Center" VerticalAlignment="Center" SnapsToDevicePixels="false" Data="M 1,1.5 L 4.5,5 L 8,1.5"/>
<ContentPresenter HorizontalAlignment="Stretch" Margin="4,0,0,0" VerticalAlignment="Center" SnapsToDevicePixels="True" Grid.Column="1" RecognizesAccessKey="True"/>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="true">
<Setter Property="Data" TargetName="arrow" Value="M 1,4.5 L 4.5,1 L 8,4.5"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF3C7FB1"/>
<Setter Property="Stroke" TargetName="arrow" Value="#222"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Stroke" TargetName="circle" Value="#FF526C7B"/>
<Setter Property="StrokeThickness" TargetName="circle" Value="1.5"/>
<Setter Property="Stroke" TargetName="arrow" Value="#FF003366"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ExpanderStyle1" TargetType="{x:Type Expander}">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
<Setter Property="VerticalContentAlignment" Value="Stretch"/>
<Setter Property="BorderBrush" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Border SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="3">
<DockPanel>
<ToggleButton x:Name="HeaderSite" FocusVisualStyle="{StaticResource ExpanderHeaderFocusVisual}" Margin="1" MinHeight="0" MinWidth="0" Style="{StaticResource ExpanderDownHeaderStyle}" Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}" FontFamily="{TemplateBinding FontFamily}" FontSize="{TemplateBinding FontSize}" FontStretch="{TemplateBinding FontStretch}" FontStyle="{TemplateBinding FontStyle}" FontWeight="{TemplateBinding FontWeight}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" DockPanel.Dock="Top" IsChecked="{Binding IsExpanded, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"/>
<ContentPresenter x:Name="ExpandSite" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Focusable="false" Visibility="Collapsed" DockPanel.Dock="Bottom"/>
</DockPanel>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="true">
<Setter Property="Visibility" TargetName="ExpandSite" Value="Visible"/>
</Trigger>
<Trigger Property="ExpandDirection" Value="Right">
<Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Right"/>
<Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Left"/>
<Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderRightHeaderStyle}"/>
</Trigger>
<Trigger Property="ExpandDirection" Value="Up">
<Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Top"/>
<Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Bottom"/>
<Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderUpHeaderStyle}"/>
</Trigger>
<Trigger Property="ExpandDirection" Value="Left">
<Setter Property="DockPanel.Dock" TargetName="ExpandSite" Value="Left"/>
<Setter Property="DockPanel.Dock" TargetName="HeaderSite" Value="Right"/>
<Setter Property="Style" TargetName="HeaderSite" Value="{StaticResource ExpanderLeftHeaderStyle}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Expander x:Name="ImageExpander" Style="{DynamicResource ExpanderStyle1}">
<Expander.Header>
<Grid >
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="auto" />
</Grid.ColumnDefinitions>
<Label Padding="0" >My Header Text</Label>
<Image Grid.Column="1" Margin="0"
Source="accounting-pic.jpg" Width="18" />
</Grid>
</Expander.Header>
</Expander>