Is there a function to put in the template to show the username of who uploaded the current image?

Not directly a template function but you can get it via the object model methods:
echo $_zp_current_image->getOwner().

Works of course this way only in context, e.g. on image.php or within the next_image() loop (album.php/search.php).

Thanks, but is there a way to make it not show the user's email? I tried to uncheck Email ID, but that didnt help.

The method getOwner() gets the user id name as it is set on the item, image in this case. If that is the mail address for some reason that is displayed.

If you want to display something else from the user account like the real name you will have to do some coding via the object model using the adminstrator class: http://www.zenphoto.org/documentation/classes/Zenphoto_Administrator.html

Thank you. It works fine for new users after I changed my settings.