Tony,
This is due to the addition of an SSL (Secure Socket Layer) certificate to the domain a while back. There is a general push from HTTP addresses to more secure HTTPS by the industry to squeeze out the bad guys. The certificate verifies that you are going to a registered site and not being redirected to another, potentially unsecured - or worse - malware site.
Any hyperlink (invariably an image, for our purposes) in a post that had a HTTP address may not work. In the case of an image, it will display a blank frame as you described. There was a plug-in installed to the forum software that was supposed to proxy these old addresses to the new HTTPS syntax, but it does not always work. Sometimes it only works for some of the images in a post, sometimes after you make an edit elsewhere in the post they suddenly reappear, and sometime they just don't.
One of the Forum maintenance tasks is to do an en-mass replacement of all the occurrences of HTTP in the database with HTTPS. Given the number of posts now in the database with pictures it is a big operation and not without risk. There has been some satisfactory testing on a replica, off-line copy of the site, but the problem at the moment is that running the same process on the actual site on the host server much slower and time-out errors has been an issue.
In the meantime when I have come across those occurrences of images not displaying in older posts, as Moderator I have been editing the post and replacing "HTTP" with "HTTPS" in the image address. Members can also edit their own posts to fix/update broken links. Any post created from about May 2017 onward already has the more secure HTTPS syntax in use.
-Doug
