I actually tried this 5 years later on Windows 8 and Warcraft Patch 1.26 and I write my experiences up here for potential other dudes out there:
I used the first way of Furby with WinMPQ but that just deleted my Background, so I got a black Background and couldn't change it. (I think something got changed through a patch or something)
Then I tried akolyt0r's way with the .reg and the local File and it worked properly.
So thanks for that hint and I hope there are some others, that still play Warcraft 3