@Annihlator
I used the map.js provided (and tried the one from Prisma), added a cart.png to the images folder, verified permissions and I copied the prefabs.xml from folder into webserver folder but there is still no icon.
I am obviously not sane, could you please provide some more detail what step I could have missed or done wrong?
Nvm, probably a rights or size problem. Got a 32x32 cart.png and made sure it had the same rights as the others and now it seems to work...