There's a little button in the second row of icons (just above the first smiley) that has a tiny picture of the Mona Lisa on it. That's the image tag button. You need to have your image posted online on a site like Photobucket first, so that you have a URL for the image. Then simply copy the URL into your post, highlight it, and click the Image button, which should put HTML image tags on both sides of the URL. Then, just post as usual and it should show up.