Tony - you might try Digital Blasphemy's website. It's dedicated to high resolution wallpaper that comes from pretty imaginative individuals. All wallpapers are available in 1900x1200 and at that are around 640kb in size, so nothing too big to fill your video buffers. It's not all astronomical in nature, as such, but very nice anyway.
http://www.digitalblasphemy.com
Cheers
Chris