iPhotoDraw is a handy tool that’s design exactly for that purpose. It helps you add annotations directly on photos.
With iPhotoDraw, you can add description text, callout boxes (ballons), dimensions, detailed images, and other annotation objects to the pictures. You don’t even need to worry your original pictures will be screwed up with additional drawings.
In fact, annotations are stored in a separated file. The application is small and very easy to use.