Ok thats easy at the top left corner of the page you will see
User CP click it.
Then on the left side of the page the first box says Your Profile click
Customize Profile
Last their are blocks and at the bottom of each block it says
Background image
you put the url their and bing your done. Dont forget to save changes also.
Remember that each block fills a precise background on your profile such as main, block borders, main borders, and etc.
Any other questions ask me.