How do I add a custom background image to my profile?

After you've logged into your account, at the top you can click on 'Browse for a file' to pick an image already located on your computer or click 'choose a stock image' to pick from tens of thousands of available stock photos.

If you want a stock photo, just enter a search term related to what you'd like your background profile to be and click search.   You should see many different options, once you find one you like just scroll down and click 'Set Image'.