You have to put the image on an external service suck as image shack. Then get the URL for the image. Then click on the 6th icon from the left (looks like a landscape photo) which will put some image markers in place, then paste the URL into the middle pf the markers i.e. between the inner set of square brackets "[img][img]"

It is a bit fiddly but is actually easier than it sounds. It's setting up the image shack account and posting the image that takes the time. If you use gumtree (see my other post re advertising) you embed the image(s) directly into the advert. Much simpler and a much bigger audience too.