Kinda the same problem, I’ve got the thumbnails, but as soon as I click on them to get the full size preview image I get “This image failed to load.” instead of the image I’m trying to use.
Happens in both Art Galleries and Client Galleries, I’ve purged all galleries from the page cashe, cleared my cashing plugin, deactivated and reactivated the plugin.
Kind of at a loss as to what to do next