There are several resolved issues that seem on-point for this, but none of the solutions seem to work. The problem is that anonymous users cannot get any images to show up in a lightbox, but if you are an authenticated user of any permission level, you can. I've read through other similar problems and it seems to come down to permissions allowed by some other module. However, I've looked at every permission setting, and the only one that anonymous users are lacking that a basic authenticated user has is the "Skip comment approval" setting on the Comments module, and activating that permission doesn't affect anything.

I can't see any configuration or permission setting that would affect how the images are presented, but I did discover an interesting thing. When comparing the source code of the pages served to different users, there are slight differences. Here's what an anonymous user gets (the image shown in the gallery and surrounding <a> tag:

<a href="/media-gallery/detail/68/1238" class="media-gallery-thumb cbEnabled">
<img typeof="foaf:Image" src="http://www.guzugallery.com/sites/default/files/styles/media_gallery_thumbnail/public/IMG_2694_1.jpg" width="450" height="450" title="View larger image" alt="View larger image" />
</a>

And here's what an authenticated user gets:

<a class="media-gallery-thumb cbEnabled cboxElement" href="/media-gallery/lightbox/68/1238" rel="colorbox-0">
<img width="450" height="450" alt="View larger image" title="View larger image" src="http://www.guzugallery.com/sites/default/files/styles/media_gallery_thumbnail/public/IMG_2694_1.jpg" typeof="foaf:Image">
</a>

The authenticated user has an extra class 'cboxElement' and the 'rel=colorbox-0' in the <a> tag. From what I've gathered, there's a module called Colorbox that is similar to Lightbox that actually looks like what is being invoked. However, I can find no controls for it and have no idea how to invoke it for anonymous people.

The site has Lightbox2 7.x-1.0-beta1, Media 7.x-1.2, Media Gallery 7.x-1.0-beta8, and Drupal is recently updated to 7.14. The other thing about this site is it was developed in drupalgardens, then migrated to an outside server. I suspect that is the root of several issues the site has, but this is the one currently most frustrating.

Any guidance is, of course, appreciated greatly.