Larer image popup is medium image not large image

Gday, I'm just starting to play with ZenCart and I'm not all that savy with this stuff. I have loaded my images as advised in other threads _MED, _LGE and in the correct folders (I believe). When I click "Larger Image" on the product Information page, the /images/medium/imagename_MED.jpg image pops up and not the /images/large/imagename_LGE.jpg image

The path for the "Larger Image" link is javascript:popupWindow('http://www.xtracool.com.au/zencart/index.php?main_page=popup_image&pID=180')

The properties of the image that pops up is http://www.xtracool.com.au/zencart/images/medium/imagename_MED.jpg

A window pops up for a very short time, then closes before another window pops up with the image in it. I have noticed the odd time when the download has slowed down that the initial window pop appears to contain a larger image, but it is very fleeting, so its a bit difficult to tell
I am using Zen Cart 1.3.8a

I would appreciate any ideas of what do do to fix this