It's taken me over an hour to work out how to load a simple .png file and use it as a texture on a box. To help prevent anyone else who might be having the same problem from tearing all their hair out, you need to ensure two things:-
* Your files must be in a directory off your main directory called "assets/Textures/[png file]". You must conform!
* Add the line "assetManager.registerLocator("assets/", FileLocator.class);". Despite what the documentation says, this isn't added internally by default!
Also, when adding the texture to the material, the String parameter isn't your own name as you might first assume, but must be the text "ColorMap".