I suggest you just pick a random model here on THW's Model Section, and you'll see a link "How to import this model". It's exactly thesame import way as you should import images.
Don't take it personal Maximilianx, but images don't need to be 256x256 (as long as it's a multiplication of 64x64 I believe), Warcraft III supports TGA and BLP as image files (maybe others, but I've never seen any others used before, so just stick with either of those 2), and you don't change the path to "War3mapPreview.tga", you remove the "war3mapimport/" part from the import path, but that will be explained in the import tutorial provided on every model's page here on THW.