There are already tons of themes featuring Mountains, Beach, Android, etc. available in Gmail to choose. You can find some beautiful themes under Settings, Themes tab.
If you have some good background images of any places you visit, your smiling baby, family photo, garden, beaches etc. you can pick one or two of them to create a beautiful Gmail theme completely on your own.
Just go to Settings, Themes and choose Create Your Own Theme. A new window with a layout that resembles your Gmail body will pop up.
Move your Mouse pointer inside the window. You can see dotted lines/frames separates main background, message body, footer etc. Click on any area and make changes to text/link/background color. You can also upload main background & footer images from your computer or Picasa album.