When you're in the window where you type your post, look at the row of icons just above. As you hover your cursor over each one, you'll get a tooltip telling you what it does. Third one from the right looks like a little picture (of a tree) and—Amazing!—the tooltip says 'Insert image'. So when you're set*, click that icon.
* To get set: You want the web address/link/url of the pic you want to post. Copy it, then paste that url into the little window that opens when you click the Insert image Icon. If it's a pic on your 'puter, you'll have to upload it to someplace/server/image host on the web where it will acquire a url you can post. Be nice and don't link to honkin' huge bloated things that suck away all Fred's bandwith and take forever to load.