No, I haven't applied any themes to forum -- just skins/containers using plain DNN skins (gray) w/ 4.8 install. Early on I did play with a skin from DNNskin.com, but uninstalled it and applied the DNN-Gray skin to the Site (unchecking Host).
After reading your message, I just went back to Skins/Container under Admin, reset both to DNN Blue from Gray, leaving both Site and Host checked this time (rather than just Site), then reset it back to Gray. It's working. However, I can see only online/offline images associated with the Forum Members with some kind of permissions (Admin, module level, forum level), not an image for regular members. At least there are no broken links. Not sure why members without various forum security roles are not showing online/offline icon, unless this is the way it's supposed to work. Meanwhile, it's hard for me to know if it was applying the skin to both site & host (from admin) or simply "resetting" the skin. And will it continue to work?
I hope this description helps to sleuth why this happened. You're hot on the trail of something here since this is the first time I've seen those images pop at all, albeit for privileged members only (is that the way it is set up?).
Thanks for looking into this, Crispy, and for responding. Deeply appreciated. -- Val