cwcgroup,

If these files are intended to be the small, medium and large images for a product, you have named them wrong.

images/Kate_Aspen/18017NA.jpg
images/medium/Kate_Aspen/18017_MED.jpg
images/large/Kate-Aspen/18017_LRG.jpg

18017NA.jpg will requre medium and large images named 18017NA_MED.jpg and 18017NA_LRG.jpg, or else you will have to rename the small image to 18017.jpg.