Open it in IRFANVIEW, resize it there to exactly your screen size and save that.
I recently purchased a Sony Vaio N350E with windows vista home premium, and i just noticed the other day that whatever image i set as my background doesn't resize to correctly fit my screen. So image is cut off of the sides and top. It does the same thing whether I'm using an image from the internet or a picture from my digital camera. The pics from the internet are usually 1024x786, and my camera pics are a real high resolution, like 2500x something, don't really remember. I've tried changing the settings from stretch to center but it is pretty much the same. Can anyone help me?? Thanks!