After two years of introduction of themes, Gmail now allows you to add a custom background to your Gmail homepage.
Main area and footer of the webpage can be customized.
Now you can add your favorite star image or a wallpaper you like the most or even the best photo of yours from Picasa web albums.
“Themes in Gmail are great — there are tons to choose from and they give your inbox a personalized look. But we’ve heard from many of you who thought it would be even better if you could give Gmail an even more personalized look and create themes completely on your own” reported Gmail on their blog
To have a good display of your Gmail page we should see the background image matches rightly with the custom colors.
On the other hand adding a huge image to the Gmail may take time for loading.
You can customize the Gmail background image by going into options and then mail settings and next to themes there you can select custom theme and can see the background option.