Well, the problem is not adding texture images to the placemark file - you could even do it yourself by changing the extension from ".kmz" to ".zip" and using a program like WinZip to add a folder of images to the archive (and changing the extension back, of course). But that would not be of any use, because there is no way to actually use these images in Google Earth. The Google Earth Client just doesn't support textures in its 3D models (yet), so any images included in the KMZ file would simply be wasted space.

Perhaps some time in the future such support will be introduced as a new feature, but since the models are only one of many functions of the application, there are bound to be other priorities.