In your onDestroy method you could try something like this:
ImageView imageView = (ImageView)findViewById(R.id.my_image);
Drawable drawable = imageView.getDrawable();
if (drawable instanceof BitmapDrawable) {
BitmapDrawable bitmapDrawable = (BitmapDrawable) drawable;
Bitmap bitmap = bitmapDrawable.getBitmap();
bitmap.recycle();
}
The cast should work since setImageBitmap is implemented as
public void setImageBitmap(Bitmap bm) {
setImageDrawable(new BitmapDrawable(mContext.getResources(), bm));
}