1. You picture must be uploaded to some other location on the internet, such as Photobucket.
2. Using your web browser navigate to where the picture is located and keep clicking until the picture is full sized. When the picture is the only thing visible in your browser window, copy the text in the address box at the top of your web browser. The "http://......jpg" stuff. Many sites like Photobucket make this easy for you by already having this text available below the thumbnail images of your pictures.
3. Paste the address to the image ("http://......jpg") into your message and surround it with the two IMG tags.
the second button on the second row of buttons in the post reply window will automatically insert these two tags and place your cursor between them so you can paste the address and be done with it. Or if using something like Photobucket yu can just copy the IMG code and paste that into your message.
If you plan to post more than one image in a message be sure to put one or two line spaces between them. You do not want to post images side by side because that forces people to scroll side to side to read every post in the forum.