I suggest creating a folder in runtime/textures and putting them there.

Then in Poser use a prop square and create a material for it by adding an image node and connecting it to the diffuse color input of the PoserSurface Node. (the one thats always there)

Then when it looks right save it to folder in the materials library as a material (mt5) .

To do the rest just change the image name on the image node and save it with a new name. 

That will give you a material that can be applied to the selected material of anything.