For the past month or so, I haven't been able to send photos to other Apple users through Messages on my MacBook Pro (Retina, 15-inch, Mid 2015), via a high speed WiFi connection. No problems sending the same photos using my iPhone or iPad. All devices are set to the same Apple ID. If I send the photo via the iPhone or iPad, it shows on the MacBook, and I can receive photos just fine. But when I try to send, the blue progress line gets "stuck" about 90% of the way. Eventually I just get an "Not Delivered" message.
I'm running macOS Mojave, 10.14.2, and the problem seems to date from installation of that OS.
Very frustrating.