You've almost got it right. But Bie Liao had to change the code a little, probably because it would force BBCode down his throat.
Don't put spaces in the brackets, as shown in the following example.
[img][/img] <-- DO THIS
[ img ][/ img ] <-- NOT THIS
Also, you have to copy the URL of the image itself, not that of your Photobucket account.
If you have any further problems, just holler. We're here for ya. =P