Where are painting images stored?

I found pop art easel paintings in the same folder, but in ClientFullBuild7.package

I'm wondering if there is a guidemap of all resources and their instances.

I don't know if this tutorial still works but you can try it: nekomimi-m0de.tumblr.com/post/174486878111/tutorial-how-to-replace-default-easel-paintings

Alternatively there are default painting replacements out there, it might be simpler to download one and then use that as your base if you don't want to track the files down yourself.