I would probably use an array to make the code less messy. Try something like this:
// UPDATE: change from String to int
int[] imageIds = new int[15] {
R.id.card1, R.id.card2, R.id.card3,
R.id.card4, R.id.card5, R.id.card6,
R.id.card7, R.id.card8, R.id.card9,
R.id.card10, R.id.card11, R.id.card12,
R.id.card13, R.id.card14, R.id.card15
};
// load the source image only once
Bitmap bMap = BitmapFactory.decodeResource(getResources(), R.drawable.vraag);
Bitmap bScaled = Bitmap.createScaledBitmap(bMap, buttonSize, buttonSize, true);
// render them all
ImageButton[] imageButtons = new ImageButton[15];
for(int i = 0; i < imageButtons.size(); i++) {
ImageButton button = imageButtons[i] = (ImageButton) findViewById(imageIds[i]);
button.setImageBitmap(bMapScaled);
}
Good luck!