That's because your texture has an alpha channel.
Warcraft 3 uses alpha channel mainly for team color, or transparency. Looking at your screenshot though, i believe this texture comes from another game, and its alpha channel is for something like specular or bump which are not handled by Warcraft 3 anyway, so you can remove it.
I guess you opened it with gimp? As far as i know gimp can only handle the alpha channel as transparency, hence this result.
Maybe there is an option somewhere in gimp to remove the alpha channel of the layer, but i don't know this software well enough.
You could also try to open it with another software (photoshop should work), or try to export to another image format without alpha channel.