The version <version>0.1.0.0</version> in reference.xml causes a dialogue popup on starting MP, stating "you are using an old skin".
All the other skins have <version>0.1.0.15</version>. I Haven't got a clue what the significance of of this is, but it is an annoying message that stops MP loading until acknowldeged. I nkow I can tick a box to make it not appear again but....
You must also call it "background.png" in lower case only. then once you have done this and every time you change the image. You need to delete these files packedgfx...bxml and packedgfx...png before you load the skin nexts.