What do you mean?
As a mod, how do I add the source to an image I've already uploaded to a wiki?
Do you mean a reference?
If yes, this should help:
It's possible they mean licensing like Fair use or CC-BY-SA. If so there isn't any standard since it seems like new wikis don't always have the set of templates defined in MediaWiki:Licenses.
If you mean description text, you should have either a "Create" or "Edit" button on the file page.
What do you think?