If you've found an image online you'd like as your desktop background, you can follow these instructions to set your background.

Microsoft Internet Explorer (Windows)

Right click the image and select "Set as Background"

Apple Safari (Windows and OS X)

Right click (OS X users can hold Control and click), then select "Use Image as Desktop Picture"

Mozilla Firefox (Windows and OS X)

Right click on a picture file and select "Set as Desktop Background"
(Windows Users) Select if you want to stretch the picture to fit or not, and optionally choose a desktop background color. Click "Set Desktop Background"