I'm using the built in auth for zoneminder and yes it does present me with a login screen. After logging in I then get the "page cannot be found" when trying to retrieve that image url. At this point I'm pretty sure this is a cert issue (especially know seeing that you have a reverse proxy setup) and that my domain needs some sort of valid cert for these images in the notification to work.
Yep, that is the documentation I'm following and it states that the self signed cert will not work for pictures in the notification or object detection. But I was under the impression that some here got it working with the self signed certs. At this point I'm not even trying object detection, I just want to get the picture of the alarmed event to show up on my ZMNinja notifications. Once I get that working then I'll move on to hooks just as the documentation you linked states to do.
So in that vain I've now set up a letsencrypt docker on the same server that zoneminder is running on and I now show a valid cert on my domain. Lets say it's mydomain.com. When I go to mydomain.com in my web browser I get a landing page that is encrypted with a valid cert from letsencrypt. I've also copied the 2 cert keys from letsencrypt docker to zoneminder's keys folder and restarted both. Yet when I go to mydomain.com:8444/zm its still shows an untrusted site, and the cert as invalid. I'm pretty sure the issue is that Zoneminder uses that base url of /zm and I see no provision in the letsencrypt docker for setting up a subdomain that uses a base url. I setup a subdomain with Bitwarden on the same server and got that working with letsencrypt and get a valid cert on its landing page but it doesn't using a base url, just bitwarden.mydomain.com which works fine. If anyone has gotten Zoneminder to work with the letsencrypt docker I'm all ears!
Thanks