The existing code assumes that the wallpaper images are the same size as the screen.

If that is not always going to be the case,  just using the screen size here may cause distortion of the image.  If the wallpaper image is 480x800 and it is displayed on a 540x960, the image will be stretched or something. We might want to crop it instead so we don't get distortion.

Also, I don't see any code here to use any of the new larger wallpapers that are added by this PR. So as far as I can tell, this is just re-using the existing 320x480 images, which doesn't seem right.