The "natty narwhal" wallpapers

ubuntu's 11.04 official desktops

Ubuntu 11.04 Wallpapers - Natty Narwhal

Various resolutions (1680<->1920 pixels)

Linux Wallpapers

 night and...

day

Size: 1920x1200
You can use them as a slideshow both in Linux and Windows 7.

Airlines Wallpapers - A night and Day

1920x1200 Widescreen

1600x1200 Standard

I found this wallpaper included in the Balanzan theme for Ubuntu...It's awesome, simple, kinda oldish...

Balanzan - A savanna wallpaper

 The wallpapers included in the latest Ubuntu edition the Maverick Meerkat.
Resolution varies from 1600x1050 to 2000x1333.

Ubuntu 10.10 Wallpapers - Maverick Meerkat