Alan Ingham:More often than not this is because the image for whatever reason has been removed by the original poster. Some users for example like to delete an image from their gallery (or wherever else the image lives) to free up space and / or the image has served it's purpose i.e. a screenshot of something. The text remains in the post unless it is modified in some way by use of the edit facility but the image which has now been deleted in the gallery is replaced with a small box containing a red cross as you have pointed out.
That problem wouldn't exist if there was a proper way of removing old posts. However, given this forums search "facility", and the inabilty to delete posts that have been replied to...
... unless I'm doing something wrong.