You can create and use custom skins in am Monday Zombies.
First creating a Level_Skin\ folder in My Documents\SteamPoweredPhénix\amMondayZ\
Now, download a skin pack and place the image in this folder.
Or You can make your own skin, just draw images:
NAME_1st.png is the thumbnail show in the title screen.
NAME_bg.png is the background.
NAME_lay.png is the foreground (and is optional).