You need the address of the picture.
I normally [right click] then [properties] and then highlight the address and copy. On here you can do the [ img ] pic addy [ /img ] with out the spaces and it should post. I know some host sites can make it more difficult, but I don't know if photobucket is one of them.
Or upload the pic on this site. its free and you just copy the link and paste.
PhotoBucket test
I pasted this in the message (without the spaces around the IMG
[ img ]http://i167.photobucket.com/albums/u134/lauren_basketballgirl/banana.gif[ /img ]