OS X's built-in screen savers come in two flavors, bundles with some config files, or a bundle with true executable code inside. We're going to grab one of the first kind, and replace its image resources with our own artwork.
Read the rest of the article for the how-to...
[Editor's note: You should be reasonably comfortable with privileges and the terminal before proceeding. I haven't tried this myself yet, but it seems very straightforward and I'll give it a try later this week.]
Now, you can probably do all of this through the GUI, but you'd have to log-out then log in again as root, so I just used the terminal app for the first part. First start the terminal, and cd to the screen savers directory:
cd /System/Library/Screen\ Savers/next type ls -l and take a gander at the files and their permissions:
total 0A quick look shows there are .saver files and .slideSaver files. The .slideSavers are the ones we want to work with. Let's make a copy of the Forest.slideSaver and use it for our custom screen saver. Because we'll soon be opening the bundle up in the Finder, we'll want it located somewhere more convenient (and safe) than the System Library. So we'll:
drwxr-xr-x 3 root wheel 264 Sep 7 2001 Abstract.slideSaver
drwxr-xr-x 3 root wheel 264 Sep 14 23:58 Aqua Icons.saver
drwxr-xr-x 3 root wheel 264 Sep 7 2001 Beach.slideSaver
drwxr-xr-x 3 root wheel 264 Sep 7 2001 Cosmos.slideSaver
drwxr-xr-x 3 root wheel 264 Sep 7 2001 Forest.slideSaver
drwxr-xr-x 3 root wheel 264 Sep 14 23:58 Random.saver
sudo cp -R Forest.slideSaver /Users/name/Library/Screen\ Savers/Replacing name with your user name, of course. Terminal will ask you for the root password; if you're the only user on the system, this password is usually just your own regular password.
If you go to the Finder and look in your Screen Savers folder, you should see your folder waiting for you. But we still need to make one more Terminal command to change the owner from root to your regular user name:
sudo chown -R name /Users/name/Library/Screen Savers/Forest.slideSaver/again replace name with your own user name. Now if you go back to the Finder and Show Info on the Forest.slideSaver file, you should see that you are now its owner and have full read and write privileges. You can now exit the Terminal, and finish the rest of the job in the Finder.
First off, you'll want to change the name of the slideSaver to describe what we're going to put into it. Just about anything should work as long as it has .slideSaver at the end.
Next, we'll pop open the bundle file to change the images within. Back in the day of OS 9, this required ResEdit or a similar app, but in this Aqua age things are much simpler. Control-click on the file and from the menu that appears select 'Show Package Contents'. A Finder window will open with one folder named 'Contents'. Open Contents, then the 'Resources' folder. In here are a bunch of jpegs. All we have to do now is drag these jpegs to the trash and copy in our own jpegs.
As far as I can tell, you can use just about any size or shape jpeg file that you like. Images the same size and shape as a desktop image work out best. When your done, close the bundle folder, select your screen saver from the System Preferences application, then sit back and enjoy the show.
You can obviously repeat this as often as necessary to create your own selection of customized screensavers.

