I don't remember where I first saw this, but you can use the "imagesize" parameter in Google Image Search to find pictures that fit your desktop perfectly. In my case I have two size-by-side 19" monitors (not widescreen). Therefore my total screen space is 2560 x 1024. A Google image search for "beach sunset imagesize:2560x1024" gives me a wide variety of pictures to choose from without having to navigate desktop wallpaper sites.