There are actually two places to add art for FreeCities. The folder you are currently in, ./resources, is not for custom art, or for the old renders. It is only for art connected to the git, and it is pushed into the HTML when it's compiled.
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 put art in the same folder as this readme, the compile will not recognize it and it will be left behind when the HTML is made.
Two different resources folders, and both connected to art. But they work very differently.