There are actually two places to add art for FreeCities.  The folder you are currently in, ./resources, is mostly for the git.  Once you compile an HTML, the HTML won't check here for art.

If you are interested in adding the old renders or your own custom art to your local version of FreeCities, then you actually want to have a resources folder in the same folder FC_pregmod.html is in.  By default, that will be ./bin/resources.  If you want any of the art in this folder, make sure to copy it over there.

Two different resources folders, and both connected to art.  But they work very differently.

P.S. make sure you don't stack resources folders, like resources/resources.  That would just be silly.
resources/Readme before adding art.md