Go to MacUpdate.com and do a search for "watermark". There's one that's an iPhoto plugin.
Or use a 3rd party editor like one of these:
Image Editors:
Photoshop Elements 9 for Mac - $79
Rainbow Painter - $30
Imagerie - $38
Acorn - $50
Pixelmator - $60
Seashore - Free
Portraits and Prints - Free
GIMP for Mac - Free
I use Lightroom, not for free, LOL