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.
With regards to the license templates, I am pretty sure new wikis have been created with the required templates for a while now. Do you have any examples of recently' created wikis that don't?
@Fandyllic Yes, that’s what I’m referring to. What should I do in the situation?
If the file has been uploaded without a license description, you can just add it manually by inserting the desired template onto the corresponding file page. Without a link to the wiki in question, we cannot provide specific examples.
What do you think?