Gmail has introduced a new feature recently to allow users to change the background image of your Gmail theme and give a personalized look for Gmail. You can set Gmail’s background to any photo of your choice from your computer or your Picasa Web Albums. Gmail already had provided the functionality to set your own colors for gmail.
To upload a background image, just go to the Themes tab in Settings option in Gmail and choose “Create your own theme”. You can then select the background images for the main area and the footer as well. You can pick from any of your Picasa images or upload a new one.