MacOS Messages not sending pictures

Occasionally, when I attempt to send a picture/photo via Messages on my iMac, I get an error message of “message failed to send” and I have no idea why. In this instance, the photo is very small (about 430 kb), and is being sent to another iPhone user. In fact, it’s failed for sending to 2 different iPhone users. I’ve rebooted my iMac and my iPhone, but that hasn’t resolved the issue. I can send text messages with no issues.

MacOS 10.15.7, Message v13.0, iOS 14.4 - connected via ethernet (and WiFi) to the internet via fiber.

Any thoughts or suggestions for fixing the issue?

I’ve seen this for quite a while on my Macs. Considering how long it has been going on, I doubt it is a network issue. I can try the resend but it usually fails too. I think it is strictly bugginess that Apple has not fixed.

This happens to me when I’m on work VPN

Messages used to not work at all on VPN, but now text only messages work fine, but images fail.

This has been my experience as well. I am currently on Catalina but it was happening before then. I had even done a nuke and pave at one point and this behavior continued.