The NExtGEN Geo plugin seems to include a second fancyBox stylesheet (twice! and for another version of fancybox) that is causing the z-index of the overlay div to be raised above that of the image div… No idea if you can disable inclusion of this stylesheet. You’ll have to ask the plugin devs.
Ok, thanks for the answer. The thing is I need fancybox/slimbox activated in Nextgen geo (it is necessary to display galleries on a map), and I need yours too for nextgen gallery.
I noticed there was a conflict between both versions if I use fancybox in nextgen geo (it gives the option to use either fancyox or slimbox, both being included in the plugin), but not if I use slimbox. Strange that it interferes with the overlay only.
Anyway it’s a minor problem, I’ll just disable the overlay for the time being, and post a message in nextgen geo’s forum anyway.