How to edit an animated Main Screen

Hi Guys, I started messing around with Warcraft again and played around on it a bit.

So, I put Undead3D_Exp.mdx as my animated main screen and wanted to edit some skins used in it.

For some reason it didnt work, if I edit them and save them in
war3x.mpq/UI/Glues/SinglePlayer/Undead3D_exp/ it just stays the same and doesnt change.
So my question is, how can I edit the skins used in there? Is there a special way I have to do it or am I saving it in the wrong folder?