Actually I've seen something like this before. One of the reasons I left my old host was that a couple of the sites I manage used Advanced Guestbook and we were having a similar issue. One day out of the blue when we would goto the guestbook, 1. it took an unusual amount of time to load 2. when it did load, the images and smilies and things were not there. Even after a refresh the smilies would not come back. For some crazy reason though if you clicked the link to sign the guestbook and then click the link to go back to the guestbook, they would load flawlessly.
The rest of web site was not effected, including the images in the frame that housed the guestbook, they loaded fine while the ones in the guestbook failed. Even after countless uninstall/reinstall attempts the images would not show up on the first load. My old host told me it was due to overloaded servers and promised to look into it and never did.
I know this doesn't provide solution, but if the message board you are using is Advanced Guestbook, you can at least know that someone else has seen a problem like this......sometimes that helps....helps me anyway.
Sorry I couldn't be more help, good luck