I think the shots you are referring to are from the volcano speleology conference now going on in Jordan.
It appears when you upload photos to Online Cavers they show up in that window widget at the bottom of the forum, regardless of whether they are actually cave photos or not. You could always upload some of your own cave photos and bump the feed.
Incidentally, NSS Online Cavers will likely not be around for too much longer, as explained
here. So the photo slideshow widget will either go away, or we will have to find some other source of photos.