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 All content on this site is provided with no warranties, express or implied. Use any information at your own risk. Always backup of your device and files before making any changes. Privacy policy info.