Andy, You can't just paste your image into the body of your message. You need to have the image on your own web site or on one of the many free image hosting web sites such as http://www.photobucket.com. Then you can paste the URL with the IMG code tag into your message.
If you use photobucket.com, once your image is uploaded to their site all you need to do is click the IMG link that will be provided under the image. When you do that the link will be copied to your computers clip board(memory) then you can place the cursor where you want the image to appear in your message. Right click your mouse and select Paste or click the past icon on the text editor tool bar above. Once you post the message then your image will show.