If you want to make desktop images adapted for your screen size, here's a trick using Preview.
Open the picture in Preview, and go to the View menu and choose Full Screen. Press Command-Shift-3 to take a screenshot, then exit full screen mode. You'll find a "Picture 1" file on your desktop; now you have your picture in full screen mode with black border around it. Very useful to have 1024x768 desktop images on the PowerBook with correct sizes.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031224165618777