I'm clearly no computer expert myself LOL, but when I've seen it done like described above, often times the picture seems to disappear somehow. I've noticed this many times while looking at older threads, the image is gone forever.
On older threads if the picture is missing it's probably because the person who posted it has moved or removed the picture from his off site storage account. The forum software goes where it's told the picture used to be. If it's not still there, no picture.