Here's a workaround for getting your favourite 800x480 wallpaper to display without bits getting cropped out. There is a disadvantage - it will look great on the homescreen but rubbish on the lock screen. For some reason, the lock screen shows a lot more of the image. What I've done is add the text "Slide to unlock" in the bottom border so it looks like it's meant to be like it.
1. Get an 800x480 image (if necessary, crop or resize a larger image down to this size)
2. Use irfanview or some other image editor to add a 200 pixel border left and right and a 320 pixel border at the bottom. Now you have a 1200x800 image.
3. Get this image onto your scroll
4. Use whatever wallpaper setting application you like to set it as wallpaper. (I used Wallpaper Set and Save).
5. Your scroll should now display the correct 800x480 portion of the image as wallpaper.
Let me know if this works for you. I am running Launcher Pro but I don't think it makes any difference.