Using photos from your Picasa account, you can customize the look of the main area and footer of your Gmail pages. have been possible for a while already.
Here's how to do it:
Click on the "Options" logo (the little gear at upper right)
Choose "Mail Settings," then "Themes" and scroll down for the last option, "Create Your Own Theme."
Follow the directions to choose a Picasa photo or upload something else, and customize text and background colors.