There is no hover effect like in core, correct, but images can be clicked and expanded. However:
This is indeed an issue. In regular topics, there is a bar below the lightboxed image with the title, a “download” button and a “original image” button. That is missing from chat image lightboxes.
Will move this topic to feature (it’s halfway between a bug and a feature request) and tag @lindsey so her team can look into it. It think it’s a reasonable ask.