you use the same url string that you paste into the image url box only you paste it into the message box instead. There is punctuation added to the beginning and the end of the string to allow html interpretation. Look at this example - http://forum.kingsnake.com/snappingturtle/messages/259.html or any other multi-picture post.
view the message box as if you were replying to it and you will see the required punctuation. Disregard the ":" in front of each line. Each string should look like this: Left arrowimg src=http......your url string....right arrow
use the preview to make sure it worked prior to posting