At this point, if you have a picture already stored on your computer in your pictures folder you will need to get it a URL address. Which means that it will need to be somewhere on the net. That’s easy. Just open a photobucket account (or similar service). You then download your pictures to that account and they will give you a URL. Hit the reply button on this message, then hit the rich editor button. It will give you a message box for you to write your messages in with all kinds of icons with which you can do all kinds of things with your message. One of those icons looks like a little picture of a tree. It is near the end of the first row. click on that. Another box will pop up, asking you for the URL of the picture that you want to add to your message and other options about formatting the message around the picture and the description. You can just cut and paste the URL to that box where it indicates.
I hope that helped.