WEll...
If you really want to know!
1. If you had initiative you would have clicked on the words 'Some BB Code allowed' and that would have shown you what to go.
but...I will tell you anyway.
Say you go to Google Images and type in chicken. It comes up with a photos of a chicken. You click on that photo and then click veiw full size image. Then you copy the url and type this into the posting bit
(with no spaces, i did it so it wouldn't come up with a picture.
Hope that it helps.