NFWar, try using Buildings\NightElf\AncientProtector\AncientProtector.blp for the path. Been working on a skin for the ancient protector as well to test an idea, decided to use the path here for a quick reference, but alas, it didn't work for me either. Dug around with the Wc3 Image Extractor and found the problem out. A simple missing "s".
Hope this helps both you and anyone else stuck wondering.