Profile Backgrounds

When you craft a badge, one of the things you'll receive is a profile background. You can use one of these as a background on your Steam profile page. To change your profile's background, visit your own Steam profile page and select 'Edit Profile'. One of the options, about midway down the page, is a spot where you can choose which background to use.

You must own a profile background to select it for use in your profile. It is possible to sell or trade away the background that you're currently using as your profile background - it will still remain on your profile page. If you change your profile background to something else, though, the background you sold or traded away can't be chosen again until you reacquire it.