キャンバスから画像を分割して、背景をそのままにしておくことができるのが好きです。結果は、以下のImageB.png（白い領域は透明）のようになります。以下に記述したコードは、asp.net C＃Webフォームにあります。
ImageA.png（背景のある画像）
ImageB.png（女性の画像、白い部分は透明にする必要があります）
ASP.NET C＃Webフォームコード。
protected void Page_Load(object sender, EventArgs e)
{
//Load image
Bitmap loadImage = new Bitmap(Server.MapPath("ImageA.png"));
//Create a canvas to work on
Bitmap canvas = new Bitmap(loadImage.Width, loadImage.Height);
// create graphic on canvas
Graphics graphicOnCanvas = Graphics.FromImage(canvas);
graphicOnCanvas.DrawImage(loadImage, 0, 0, loadImage.Width, loadImage.Height);//Draw the graphic to the canvas
//*****REMOVE IMAGE FROM BACKGROUND ?????????????????????? ********/
#region Output image on screen
MemoryStream msOut = new MemoryStream();
canvas.Save(msOut, ImageFormat.Png);//must leave as png to output as png
canvas.Dispose();//Dispose the canvas
Byte[] BitmaptoBytes = msOut.ToArray();
//convert bitmapholder to byte[] - ended
BitmaptoBytes = null;
Response.ClearHeaders();
Response.ContentType = "image/png";//to product a better jpeg we have to use this because event if using the codec to do it it still doesn't look good http://msdn.microsoft.com/en-us/library/aa479306.aspx
//disable 1 line below to prevent download from viewing
// Response.AddHeader("Content-Disposition", "attachment; filename=" + ProductImage.Substring(0, ProductImage.Length - Reverse(ProductImage).Split('.')[0].Length-1) + ".png");//changing the file name extension
Response.AddHeader("Content-Length", msOut.Length.ToString());
msOut.WriteTo(Response.OutputStream);//Sending image out
Response.End();
loadImage.Dispose();
#endregion
}