Your photos have to be hosted on another site such as photobucket. Photobucket has the IMG code directly underneath the photo, so you can cut & paste the whole code into the message. Make sure you have all the end brackets
.
You can also insert an image by clicking on the little mountain icon in the bottom row of the message toolbar that you see when you are creating a post. A window pops up asking you to enter the URL for the image. Cut & paste the URL there (no IMG or brackets).