As already said, either add as attachment or link to a hosting site
The message you just posted, or any message, just scroll down when writing it and underneath you will see the section with 'If you wish to attach one or more files enter the details below' writen on it, which contains the 'browse' button, press that and navigate to and double click the image you want to attatch, click add file
Screenshot of message page with browse button under main message text box...