you will likely need to upload the photos to a host like photobucket or imgur then past the link to the photo here.
in the message edit window there's an icon that looks like a picture above. upon clicking a popup window appears prompting "Please enter the url of your image:" in which case you would paste the link and click okay.
im sure i probably oversimpified the process but its a good starting point for you to look at whats involved.
"I am Groot", the faithful protector