I inatalled this on a cart couple of days ago and have been reading trying to find an answer to this.
The extra images seem to add and show up just fine. Now however, the main image for the product shows the red x for the missing image.
the link to the image is images/medium/IMAGENAME_MED.jpg but there is no image in that folder for this.

Interestingly when I click on that miasing (or the link larger image) the large image appears.

What is missing to create the _MED.jpg image?