Unable to send photos via Messages
New 16" mbp...and cant send photos via Messages. Signed out of imessage and back in. No avail. Quite frustrated. Much thanks for the help. No problem on the desktop.
MacBook Pro with Touch Bar
I've experienced the same issue sending images coming from an external disk. The only working workaround, as you already mentioned, is to move the photo on the Desktop and pick it up from there: