Whether you use tags like the following or create a new bbcode, you will need to add the tags to your string to get it to automatically convert. For example if you use the code below and just wrap the url with img tags it automatically converts the image for you. What am I missing?
HTML Code:
[img]http://nt2.ggpht.com/news?imgefp=DiVXajp1xvQJ&imgurl=images.alarabiya.n%20et/large_6277_66683.jpg[/img]