While this is something that Chris may want to address, I also administer a vBulletin based forum site and feel like I can chime in. When someone links an image file to a post on the forum from a 3rd party site such as Photobucket, etc., there is no way for the forum site to retain that image if it is deleted from the 3rd party site. It will just show the image placeholder as you described above. The only way to assure preservation of any image posted on the forum site would be for the image to also be hosted on the forum site. Sadly, that isn't feasible due to the huge amount of server space required to host all of the images that get posted to a busy forum like this one.
It sucks but that's about all that can be done.