You can just include the image url ( i.e. http://www.classkids.org/images/donatelife.gif) in your message.
Or you can upload an image(this option has to be enabled by the administrator) by clicking
on Click here to choose your file on the post form. This will launch a remote
window through which you can specify the file extension and browse your disk to select the file you wish to
upload. Once the file is selected, the name of the uploaded file will automatically be inserted in the body of your message. All files are randonmly renamed. Keep in mind that there is a 25 KB size limitation to file upload.