Essentially, it seems that the tag is actively checking if the URL ends in a ".jpg" or ".png" or other image format, and if it doesn't, instead of behaving like an image, it behaves like a link and displays an anchor with the URL instead of the image itself.
For instance, in this topic I just created:
http://www.oldunreal.com/cgi-bin/yabb2/YaBB.pl?num=1521812290
I had to add a fragment to the end of each image URL as "#.jpg" and "#.png" to fool the forum software into thinking that it was a "normal" image URL in order for it to work, although any URL whatsoever can be a valid image and should be accepted as is.
The extension check should not be made at all since what defines the type of a resource in any web service is the MIME type which comes in the Content-Type header in an HTTP response.
