Gmail Skins is a site with nice pictures you can add to Gmail as custom themes. The site was built by Greg Bullock, from the Gmail team.
You can sort photos by date, title or popularity, select a category like macro, water or nature, find your favorite image and set it as a Gmail background. There are two options: light themes and dark themes.
The site also lets you upload your own photos to the gallery. "Only upload images which you own.
Only images which are 1) mega-awesome and 2) sufficiently large will be published."
{ Thanks, Greg. }