1. Open Display Properties in the control panel. That will tell you your screen resolution.
2. When you find a wallpaper you like, see if it's the same resolution (1920x1200, 1280x800, regular or widescreen...examples) and choose the same resolution or larger, keeping the same ratio of width and height).
3. Some wallpaper sites would be http://www.nexus.com ... some of these will even fit the wallpaper to your screen resolution.
Give this a try. Go to Nexus, choose a wallpaper and click it, it should fix it to your screen size. Then right click that image and save it to a wallpaper folder under, say, My Pictures.
Then, Navigate to where you saved it, right click on it and select "Set as desktop wallpaper".
If you have more questions, please don't hesitate to ask. I'll try and cruise by to see how you're doing.
By the way, welcome to Wincustomize...hope you decide to stay, rrew