Till now Gmail offers various themes to get personalized look and for customization of your Gmail (Android Theme is the best one in my opinion), why not go ahead and create your own theme with your own background image or picture chosen from Picasa albums, Gmail team think so, yep! you can now create your Theme for Gmail.
1. Visit Themes tab in Gmail settings and choose “create your own theme “.
2. Select the background image from main area in footer, select background color
3. Click “Save” button to see your own created theme for Gmail. If you’re not satisfied with the theme click “Reset” button to delete all changes .
4. Your Gmail will get classic theme again, not the one you’re using earlier. [Via] Gmail blog