So I'm finding an incompatibility between W3 Total Cache and lightbox plugins.
The one I was using originally (the old Lightbox 2, no longer in the plugin directory) was being a little glitchy-- clicking Close would not close the box-- so I tried a new one.
And then another. And another. But haven't gotten a single subsequent one to work yet. They're even worse than Lightbox 2-- just open the image URL in the browser without any lightbox functionality at all.
Does anyone have a lightbox plugin working on their site with W3 Total Cache? I would love a recommendation!