From the 5E module Post

Here

"Only include work own work, OGL/Creative Commons material, or material that you have permission to use. Do not violate any copyright laws with the module data you post here. As a reminder - getting images from an internet search usually breaks copyright: just because information is freely available doesn't mean that it isn't covered by copyright. Thanks."

The following deals mostly with art, but lends itself to other things as well.
These are just IMHO.

Ask the maker if it is alright to use, if they saw yes, please put their info in the credits, web page where they show the work, and so on.
Also ask if they want it used only for this one module, or open it to others as long as credit is given.
Let them if they wish to write their on credit blurb, or to Ok the one you write.

Many folks are happy to allow use of their work for the credit.

The worst they can do is say nothing, then No.

Then you have to find another pic for the module, but many times they say yes.

I would have a little note pre-made to explain how FG uses art, and where the credits are found, things like that so they know what they are getting into.


Good luck on your Modules!
Lee