Hi Dave,
I'm not sure of the specific requirements for this link to show up, but it is possible that the original image used for your header/gallery image needs to be at least a certain size in order to make the "Enlarge" link show up below the header image area. I know that our “Click to Supersize” link will only show up if the original images are larger than 400 px x 300 px and it is possible eBay does something similar.
If you are still looking for an answer to this question, I recommend contacting eBay's customer support group for further clarification.
-Mike