Gmail today(June 19) rolled out a new feature that lets you set any picture as your background image in Gmail. You can select photos from Google Plus or simply paste any image [abbr title="Uniform Resource Locator"]URL[/abbr] or you can upload your own photo.
To create a new theme→ Head to Settings→ Themes→Select Light or Dark and Upload your image.
Currently this option is not available for every one, it will hit your inboxes with in the next couple of days. In the meantime watch the official video.
Via→ Official Gmail Blog