Photoshop and Aperture are professional applications, one the gold standard image editor and the other one of the top DAM (digital asset management) applications. iPhoto is a entry level DAM application with basic editing features.
As Terence mentioned there are a lot of 3rd party applications that can be used in conjunction with iPhoto to add watermarks.
If you have Photoshop or Photoshop Elements for Mac then you can use it from within iPhoto for that very purpose and more.
TIP:
For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto (iPhoto.Library for iPhoto 5 and earlier) database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've created an Automator workflow application (requires Tiger or later), iPhoto dB File Backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. It's compatible with iPhoto 6 and 7 libraries and Tiger and Leopard. Just put the application in the Dock and click on it whenever you want to backup the dB file. iPhoto does not have to be closed to run the application, just idle. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.
Note: There's now an Automator backup application for iPhoto 5 that will work with Tiger or Leopard.
