For a picture to look "right" on your desktop the picture needs to have the same number of pixels (across and down) as your monitor resolution.
To find your monitor resolution select on the desktop with the RIGHT button, then choose properties, then the settings tab.
Note the numbers in the screen resolution field (1280 x 1024 or whatever yours is)
Now go to a "wallpaper" web site and download a picture matching your screen resolution size.
For example this site offers pictures in all sorts of sizes:
http://interfacelift.com/wallpaper/
In fact there are some GREAT pictures on there.