Two ways to do it...
1) if the image is already online somewhere (i.e. on your website, for instance) then you can use the tags, putting the http address between img and /img tags. Or you could make a link using the url and /url tags.
2) if the image is not online but it's in a web-compatible format (.jpg, gif), you can "attach file" (see below where you type your message) and it should display in the message. There's a file size limit.