It doesn't do anything to the image... should it?
You might want to change the grabpixmap to use the same x,y as the drawimage (i.e. grab the image rather than the whole screen) and use imagewidth/imageheight to get the extents.
You might want to change the grabpixmap to use the same x,y as the drawimage (i.e. grab the image rather than the whole screen) and use imagewidth/imageheight to get the extents.