You can just do the "manage attachments" thing as already mentioned, but i think it limits how big your pics can be, which might be why it won't let you do it ???
Also i think once you have posted a pic in a thread, it won't let you post that same pic again in a different thread until you have deleted it from the first thread ??? i think that's how it works ???
Once you have your pics on photobucket you put your mouse/cursor on the pic and a drop down section will appear under the pic, you click the little box under the pics you want to use and then you can either copy paste the "IMG" link under the pic if you just want to post up one pic, or if you want to post up multiple pics you can click on the link at the bottom right hand corner of the page that says "generate HTML and IMG code" and copy paste the contents of the "IMG" section straight in to your thread.
Woody