This is just a really simple guide for getting a simple background if you want one. First you're going to need multiple tabs, if you don't know what tabs are then i hate you, with the tabs go to google and then switch to a new tab for your edit profile and then go to the about me section and put this code* in some where.
next go back to google and do an image search for any image that's appropriate and cool/stuff you like. go to the image at full size(don't make it too big >.< ) and right click then find the properties option and click on that. a window should come up that says image location, copy the url then place the url here**