Google rolled out themes in Gmail in November 2008 — and as on date there are tons of themes available to choose from to give your inbox a personalized look. But to help you give Gmail an even more personalized look and create themes completely on your own.
For a while, you have been able to set your own colors, now starting today you can customize your Gmail 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.
[Source: Gmail Blog]