It sounds to me like you have your desktop set to tile and if the image is just a bit smaller then the window you can end up with getting a realy bad tile seeing very little of the wall paper depending on the size I would check that first if you right click on desktop and personalize click desktop background usually there is a radio button under the place you pick wall paper make sure its set to center or fit to screen depending on the image.

I usually perfer center unless it is really big which will often crop off the top or bottom of your image.