If you're on photobucket on a web browser you should see a set of four boxes with links in them on the right that says "SHARE LINKS". There is one that says IMG, that should be on the bottom. It should look like this:
If you click on the link in that box it should say "copied". You can then just post it into the thread.
Alternatively, you can click on the one that says "Direct" and then post that link in between [I M G] and [/I M G] without the spaces.