After enabling themes and setting a background color for GMail, now Google let you decide if you have better background image and want to customize it for you GMail. It's called Custom Background Image Themes.
It's a very pleasant features as might have better images and we want to use it but we are limited to Google's image themes. Now, we are free to customize it as we like.
Thanks to GMail engineers
Take a look on this screenshot example by Google Engineer