okey first of all why is this in the Rihanna spot??????????. Anyway you go to a picture you like ,right-click it and you see set as background and click that . If you want it to fit the whole screen of your computer go to desktop background and there is 3 types one with the picture big ,the other halfed and the other is small . Go to the one that has it in full screen and presto thats it. I tried to make it easy for you
Good luck

yah and why is it here not to be rude by the way