How do i change the wallpaper of my dell inspiron mini without downloading anything?

i've tried right clicking on the picture i want and selecting set as desktop background but it doesn't work.then i tried right clicking and looking for the personalise word (as they said in many other websites i asked) on the little thing that pops up but it doesn't say personalise.Oh and my laptop is a windows 7 starter, Help? please and thank-you. : )