How do I create zip-file from Collada 3D-model in order to make it keep its texture when imported to OpenCities Planner?

I've been downloading some simple tree models for free from the internet. They all look good enough for use in a small project made in our model of the town center in OpenCities Planner.

Problem is they all loose their texture when bringing them into our plan. I've learnt that I should create a zip-file including the texture with the model itself but after doing that in several different applications I only see them fail when imported in OpenCities Planner.. They all appear in the project missing their surface/texture.

Does anyone know how to handle such simple 3D-objects before being imported - and make them keep theír texture?

Regards, Anders Theodorsson, Ludvika Municipality GIS-office