Members are able to attach files (images/etc) to their posts and Private Messages (PMs).
Each Member has an upload allowance, and once this is exceeded cannot make any more attachments until they've freed up some space by deleting old attachments. You can manage your attachments to posts using My Controls->Options->Manage Your Attachments.
What is not so obvious is that PMs you send, including their attachments, also count against your upload quota. However attachments to PMs don't show up under Manage Your Attachments, although the amount of space they're taking up is shown. If you delete the PM from your Sent folder this still doesn't free up the space, as the system keeps the attachment so that the recipient(s) of your PM can still see it. The attachment is only deleted, and your quota freed up, when both you and all recipients have deleted the PM (or archived it to a PC).
Because of the way the system works you should therefore think carefully before sending PMs with attachments - you're relying on the recipients to delete the PM when they've finished with it, otherwise you're permanently committing some of your upload allowance! If you need to send a PM with an attachment then you should make it as small as possible, and ask the recipient to delete the PM when they've finished reading it (or archive the PM to their PC). ...