It's because admin removed attachment.
When someone add a picture from it's computer to this forum trough a attachment. the picture is saved on the server the forum is on, and we/they don't have the space for all the picture that is getting uploaded in attachment. (The server SB is on have limited space)
For the record, the files were actually never deleted off of the server, they were just made not-visible, but the names were lost. I've got a copy of the files around here some place. The names that they did keep included the account number of the user that uploaded them.