I can't change my desktop background. when i try to select a group from the list after right clicking on the desktop and clicking 'properties' 'desktop' and in the background area it has 'none' at...

I am having problems with the backgound of my computer. The backgound is blue and when I right click and click 'properties' and then 'desktop' the option always chosen is 'none' with a circle and a...