I think the only way would be to upload the photo to a hosting site on the Internet and the include the link to the photo in your message. For example, I have a photobucket account, so I would upload the photo from my computer/phone/tablet and then copy and paste the link into the message.
Not sure if this helps.
The other option is to click on the link to My Gransnet at the bottom of the page and click on the Share Photos link. You can start a new gallery to share there. It's not adding a photo to message, but it is a way to share photos on GN.