There are 2 usual reasons and 1 more that may help.
1: The user may have left the forum and taken there photos with them, or deleted their photo hosting account. So they're never going to be seen.
2: The Photobucket issue where Photobucket wants $400 per year from everyone who wants to post photos on sites like this. Again, never going to come back.
The last one is potentially your forum settings. If you go into your account (top right of every page) and click on the 'edit forum options' button, scroll down til you find the checkbox which says "show images" in the thread display options. Make sure that box is checked and you'll be able to see all the photos that are available. It won't help with the first 2 reasons though.