I was having the same issue with my new MAC Mini. Photos would not upload in Messages.
Super frustrated. Finally I called AppleCare and the first question the tech asked me: Are the pictures on an external drive?
What? Why would that matter? But yes in fact they are - I keep all my photos on an external USB 3.0 4 Tbyte RAID 1 drive - for redundancy purposes.
Ok, please try taking the photos in question and coping to MAC Mini SSDD drive and then upload.
Sure enough that "fixed" it!
Mac OS Catalina MESSAGES won't upload pictures from external drives. You have to move them to the MAC's internal drive/ssdd first.
I'd been struggling with that issue for months. That is all it was.
I now realize why it worked on occasion. It worked with the pictures were on the SSDD - typically after having dumped them off my Iphone.
Apparently Messages can't resolve addresses to external drives.
Hope this helps someone else!!