Google just announced custom background image themes:
Themes in Gmail are great — there are tons to choose from and they give your inbox a personalized look. But we’ve heard from many of you who thought it would be even better if you could give Gmail an even more personalized look and create themes completely on your own.
For a while, you’ve been able to set your own colors, and starting today you can customize your inbox with your own background image too.
Just go to the Themes tab in Settings and choose “Create your own theme.” There, you can select background images for the main area and the footer.
You can pick from any of your Picasa images or upload a new one. Enjoy!