Google just announced that Gmail users can now create customized themes for their inboxes.
Google has let users select from various themes for about 4 years, but until now, you couldn't just use your own photos (or any from the web).
The offering, of course, comes with Google+ integration, as it allows you to select photos from your Google+ account. If you use the Instant upload feature that comes with the Google+ mobile app, you should have instant access to any photos you take with your phone.
As you can see, the selection interface is very reminiscent of Google+ photo viewing:
If you want to use something from the web, just paste in the image URL.
You can also set your theme to either light or dark.
The custom themes feature will be rolling out over the next couple of days, Google says, so if you don't have access to it right away, be patient.