First you need to store your pictures somewhere online, be it a personal website, Photobucket, Flickr, wherever.
Then you need to put a link to the picture in between image tags, which you can either type out as (img)link.jpg(/img) (but replace ( ) with [ ] ) or, you can hit the button right above the typing window that looks like the Mona Lisa:

(Quote this post if you want to see how that was done, so you can see the message as I typed it)