First, you need to find a hosting site, like Photobucket or one of the many other sites. Then you need to find the URL (web address) of the photo. It should appear in the address window. But photobucket and some of the other photo display sites have little apps, so you can click on a box and it copies the address into your scratchpad. You need to click on "direct address."
Then, you start a message, and click on "rich formatting". This shows a number of icons along the top. The single frame of photo film icon is the still picture link. When you click it a box occurs, with a window asking for the URL. You paste in the address you copied, (or that was automatically copied by the app). Then you click on insert and the picture should appear in your message.