can' see any picture

when ever i click on some pic i got message:

vBulletin Message
Invalid Attachment specified. If you followed a valid link, please notify the administrator


Why ?

is it ALL photos or just some? sometimes people remove photos. therefore its no longer there and you will receive that message

i get that error if im not logged into riu. Or if I was logged into a thread and then it timed out. So even though it looks like im logged in im not. usually if you login and go back to the page and open the attachment it works.
Ive also noticed most times if you see a link that says attachmentblah blah instead of seeing a thumbnail then im not logged in.