mybmp = (Bitmap)pictureBox1.Image;

英语翻译
This flag is set when the bitmap has an Alpha channel (that is,per-pixel alpha).Because this API uses premultiplied alpha,the red,green and blue channel values in the bitmap must be premultiplied with the alpha channel value.

英语翻译
you do not need to account for separators in the bitmap because this function does not assign image indexes for separators.if your *** has buttons at positions 0,1,and 3 and a separator at position 2,the images at positions 0,1,and 2 in your bitmap are assigned to the buttons at positions 0,1,and 3,respectively.