I know this must be frustrating.
If the images are remotely hosted (e.g. uploaded at Photobucket, Image Shack, Flickr, etc) and inserted into posts via a link, we have no control over what happens if they should be deleted from those websites.
That could be what has happened to the missing images. I think this is especially likely if they are showing up as blanks in older topics where the members are not active here any more.