This skin pack is no longer in development? =(
Happy news: It is in development, BUT due to limited time and my holidays it is slow, but I will release a new version when it's ready.
Yep, their just .pngs. The icon is 52x52px large and the title-icon/bar/whatever 305x57 px large.
Feel free to make your own icons. The more the better so everyone can choose which ones to use.