The FSM forum does not host photos. That is, they do not store the photos on their site. You must find a third party to host them, or else post them on your own web site. A lot of us in these forums use Photobucket, a free photo hosting site, but there are lots of others.
Then, when you compose your message, you must use the rich format mode. There is an icon in the toolbar when you are in that mode that looks like a photo print, right next to the two-frame icon that represents video. Clicking that opens a window that prompts you for the web address of your photo (the URL). You copy that address, and paste it into the window that is asking for the address. If you use Photobucket, there are several addresses displayed- you want the direct address. When you then click on OK, the photo should show in your compose window.