So the first thing to do is find a picture you like on the internet then right click the mouse. There should be a number of options that come up but you need to click "copy" this lets you copy the picture without taking the picture away, then push the start menu button on your computer And find the "my pictures" option it should be on the right hand side and double click it, it should open quite quickly after you have clicked it.
You then right click the mouse and click the option "paste" then again right click the mouse on the picture and click the option "set as desktop background" then exit out of everything and check that your picture has changed. You should then be finished.