I believe you can, sort of.  There is a way to insert your own jpgs for the ones that are included.  You would need to be somewhat good at a graphics program like GIMP or photoshop though.  He gives a template to use for guidance.  At least he does for the traffic jam addon.  I believe he does with the basic M.O.M. package as well. 

 

What you won't be able to do is to change it inside Poser.