My site is working just fine in all other aspects, but when I left click on a reduced-size image to display the full image, I get a 403 Permission Denied error. I am using Bluehost if that makes a difference. Any thoughts?

Is it possible that you have selected no access for your protected image options? Perhaps you have assigned a password for viewing them?

"no access" is not selected. It's currently unprotected. the gallery is private, but the user I'm logged in with has full rights to include view full image. I also set it to public to test if that was a problem, and the same thing resulted.

I also noticed that the .php url and the .jpg url (the full image) differ in that the full image url places an albums directory into the url. For example:

mainsite/Photos/2012/20120718/20120718_094118.jpg.php
mainsite/albums/Photos/2012/20120718/20120718_094118.jpg

When the protection of the full images is "unprotected" the URI generated is directly to the images in the album folders. A forbidden access for this would be caused either by the file permissions of the images (or folders) or by an .htaccess preventing direct loads from these folders.

About your url question also see: http://www.zenphoto.org/news/my-pictures-are-in-zenphoto-albums-myalbum-but-the-url-says-zenphoto-myalbum-it-s-wrong-