Yes, it is possible to add a photo to a message, but only if you use the phone app, @Keith500 .
If you do not have the phone app, you can use the temporary guest email address generated by Airbnb to send any photos or attachments. Those sometimes go to the guest's spam folder, so just mention in the messaging that they will be receiving an email from you and have them confirm they received it.
You can find that temporary email address at the left of your messages with the guest. Scroll down until you see "Email guest". It is the long string of letters and numbers with something like @guest.airbnb.com at the end.