user + settings for editing picture captions in frontend

[ngc]

Hello,

I would like to give an user the rights and possibility to edit picture captions without logging in into the backend/administration.
So that the only login required is via /index.php?option=com_users&view=login

But I have no idea of the whole process... do I need to create an extra user in phoca gallery, or is a joomla user of the "Editor" group enough? And where can I set that this user can edit the captions of all existing categories/images? And what settings do I need to enable?
Many questions unfortunately.

Maybe someone knows a how-to somewhere?

Thanks a lot!