Hey Bart,
As mentioned, place your image URLs between the image tags. In order to get those tags to appear on any message you post, click on the icon that looks like a small picture in a frame, just to the right of the spinning wheel icon (it's above the smiley/emoticons row). Here's an image of the icon:
Then cut 'n paste your url between the image tags, as Mike showed, and you should be good to go!