if you have a picture published on the internet, click the insert image icon above the message field and paste or type the url of the image in the field provided then click OK.
If you have an image on your computer see the table below the message box that says Additional Options. Click on the button that says Manage Attachments. Click the browse button and select your file. Click Upload. Once done, click close this window. After you post your message, everyone will see a thumbnail that they can then click on to enlarge.