I would like to display a link on image in a gallery, linking them to say shutterstock, or someother stock. Is there a plugin or someother way to do this in zenphoto ?

Put the link in the image's description? Also, available and known plugins are listed on the extensions section.

is there any other way except putting it in the description, e.g. can it be entered into custom data and displayed as a link at a certain place on the page ?

Sure or any other field available but you of course will have to modify your theme used to do so.

what function should i use to print the custom data field for images ?

http://www.zenphoto.org/documentation/functions/_template-functions.php.html#functionprintImageCustomData