Have two windows open: 1 is your Photo bucket album, other is the New Post in this forum.
One the Photobucket page, find the IMG box and click on it

It should go yellow and say 'Copied'
Then you into your message thread and paste (Ctrl V) into the message
It should say somthing like this:
[IMG]http://i1011.photobucket.com/albums/af234/Jamo_kiwi/Z%20Oddments/How%20to.jpg[/ IMG]
Except I inserted a space towards the end of that URL to stop it turning into the image
Preview the post to check it has worked
Cheers
James