just use the "attach" feature (paper clip looking thing above the text box) when your making a post. It will let you upload a pic from your computer. or if the pic is online somewhere just right click and go to properties, copy the "url" and then clicki the little picture feature button and paste the url.
Oh, then you have to upload it to a picture hosting website like www.photobucket.com or www.imageshack.us. It will then provide you with a URL, but unless you intend to store photos online, you may not want to do this.