To use this new feature, please visit the section of Gmail settings and click Themes and columns. Then the next item you select Custom Themes, and click Next noticed the link "Change your background image" below. Then the user can choose the photo collection on their Google + or paste the URL of the image to choose as wallpaper for your own Gmail. Google also added the ability to browse and search for high-quality image to set as the theme. it can be due to network or how it should be possible that some users probably will not get this functionality you can patiently wait by Google will apply new features for all users in the near future to come.