{img}image url goes here{/img} and the {} get replaced with []
There is also a button up near the top when you are creating a post that will automatically create the image tags, then you just insert the url.
Or, for images on Photobucket, when you look at the photo in your Photobucket album you should see several "Share Links" off on the side. One of those says IMG before it, just copy all the text in the window and paste it directly into a post.
