You need to upload the picture on a hosting sight. For example (photobucket.com) Where you need to register an account, and then you simply click the button that said 'browse' or 'upload' and then search yoru derectory for the picture and click 'ok'.
Then when it finishes loading it. You can scroll down and see a small tuhmbnail version of your picture. Below it are three white lines with text in them. One says 'url' which if you copy the text fromt hat one and past it in your message, will show a link that they can follow to the picture.
But if you copy the one that says 'IMG' then when you paste it in your message, it will show up as a picture.
Give it a try in this topic.