Windows xp desktop backround change not functioning :/

[Choobakka]

Hi this is another post by me, choobakka and im asking for one more thing hhere on the forums
as most ppl know if u want to change your desktop backround u right click then goto properties
and desktop and just click and choose wat theme you want. but I can't change my background to a pic or a theme i can only change the color. My background options won't light up and i cant even click one of the themes.......any tips?????.... i cant see a theme which is my problem, as desktop background either.