I've been trying to import a "assassin" skin into the world editor fot TFT, but it always fails. I'm usingthe path that's in the WCIII image extractor directory:
units\creeps\assassin\assassin.blp
That's definitely the correct path; not knowing many of the bugs with the editor, the only thing I could suggest is re-saving your .tga (or .bmp?) as the .blp and importing again.
i dont know if im right, but i remember some1 saying something like "u have to close the map and open it again. That should apply the skin and then it shows correctly" or something like that.