I am somewhat familiar with the program, and have added an additional Library into ArchiCAD once before, but I downloaded some additional textures from the Object Depository. They are just jpegs, and I figured I could put them all into the Textures folder, but that doesn't work.
So my question is, how can I get Archicad to recognize these new textures?
You did right.
Now you must reload the library for Archicad to recognize them.
You might notice that one of the longer processes Archicad undertakes when opening a file is to review the library elements. When you add new ones the entire library must be scoured for potential conflicts - especially duplicate names - whenever new things are added..