It's pretty easy, Glen:
Just type your text, and where you want the picture, click on the little box along the top that says "Img", a little img will appear, type in the link to your picture (has to be available on the internet, I believe, and then close the photo link by clicking on the img box again, thus:
In preview you should see the picture appear!
You can also add it as a file attachment, using the box below the post a reply box.
Hope this explains it well enough.
Jim