Here is three best Wallpaper for Ubuntu 10.04 and you can also download the whole collection FREE from the link given in this post.
1. JK Bridge at Night
2. Flower Drops
3. Reflecting Clouds Near Beach
If you want to download more wallpapers for free then link is given below.First one zip(No password is required) file contains 4 wallpapers for all in different resolution while second zip(Not protected by password) contains more than 140 wallpapers(Including the above three wallpapers).