Link: HubbleSite - Wallpaper

Wallpapers resolution selection offered is [typically]

640 x 480
800 x 600
1024 x 768
1280 x 1024
1152 x 864
1280 x 768
1280 x 800
1280 x 960
1440 x 960
1680 x 1050
1920 x 1200
2048 x 1280
2560 x 1024

There are currently 81 wallpaper images to choose from.

The Crab Nebula [2560 x 1024]