Ajeh - see answers to questions below:
"Reload the /includes/pages/popup_image/. files ... does it work?"
My path is /includes/modules/pages/popup_image/ so I renamed the popup_image directory to popup_image_bak then uploaded the original (unaltered) popup_image directory from the zip file. There was no change.
"Switch to Classic ... does it work?"
No it doesn't work with the Classic template either.
"What version Zen Cart?"
1.3.0.1
"Is this a clean install or an upgrade?"
New install, assisted by Sashbox.
NOTE
After launching this site (July 2006) I had Sashbox copy the entire ZenCart and database to another directory so I could build another store with the same functionality for the same client. The pop-up images still work on this site, and although it is under construction you can see it here: http://showidentity.bestpriceuniforms.com
THANK YOU FOR ALL OF YOUR HELP!!!