There are lots of Gmail themes available but when you want to set your own image as a theme here are some steps, login to your Gmail and go to Settings -> Themes and from here go to the bottom of the page where you can see a link – Create your own theme. This will help you create your own themes by adding your favourite colours and also set your own image as a theme.
Click on the link – Create your own theme, this will open up a popup as shown below,
You can change any colours you want and save the theme, or just add an image as background. To do this click the select link as highlighted in the screenshot above and add an image and save the theme to see it on your Gmail page.