If you still a true Gmail fan and you like the inbox web interface more than connecting your account using an email client software, then you’ll be happy to know that Google is updating its email service and adding two new custom theme options. Now you can set your background image in Gmail and here is how:
Just sign in to your email account, navigate to the themes settings, scroll down to Custom Themes and choose from Light or Dark presets. You can select from a handful of images by clicking the Featured photo section in the left menu (see above image). But it is obvious that you want to use your own, so of course you can upload your own images, set backgrounds using Google+ pictures, or you can even use an image URL, then Gmail will do the rest.
According to the company this new feature will help users personalize their inbox making it less boring and more practical, and also will help to quickly identify between multiple Google email accounts.
Source Official Gmail Blog