They really do need to change that , Most forums i use don't even allow you to link pictures from Photobucket ect (or via weblink). You have to Actually use the Attachment option that this forum lacks and upload it to the forum from your pc.
Reason for that , Later down the road when say Photobucket or what ever is dead and gone the pictures in the post will still be there since they were upped to the server itself and not reliant on some 3rd party hosting site.
and really it doesn't really cost much to add some storage to the server for this purpose.