There are lots of reasons why that can happen. Usually it's because the image was deleted but the particular hosting service doesn't return a token "not found"/404 image instead. I've found that Imagebam, Imagevenue and Fastpic.ru are hosts which do that, whereas Imgchili, Imgbox, Pixhost, Imgur/Imgleon, Picsee, etc. are hosts that return token .gifs like red X-es, "Image removed/not found" boxes, and the like.
It can also depend upon whether the poster has included a link to the host's retrieval protocol with a host-generated thumbnail, or if they are trying to hotlink direct to the hosted image (or, similarly, include the image inline in the post). In the latter case, hotlinks sometimes cause a hosting server to return a token error .gif, but not always. Hot-linked pics from Twitter or IG are also instances where you're not likely to get any visual confirmation of "image not found".
The only way to be sure is to view the raw HTML source of the post (not the BB code). I've found that I can sometimes "massage" a direct link in the post back to a retrieval invocation URL appropriate to the hosting site, then invoke that natively in a browser tab and see whether the image or token error .gif comes back. Hope that helps...