In the posting box (where I am now typing this), you will see a series of icons, starting with a "double capital A" on the left side. The second icon from the right in the series (which is yellow-ish on my Macintosh Mac Book and has a very small picture of a couple of mountains and the sun in it) is the "picture posting" button.
In order to use it, you have to first ensure that the image that you wish to post is itself available on line. To do this, first upload your picture to a service like Imageshack, then get the online address of your photo, hit the "picture posting" button, paste in the address for your image, and there you go.