In your case you need to add the image, not the URL. Try something like this:
- service: notify.<yourdevice>
message: Here is a test image
I save the image to disk first (use the folder
/media/Cameras/cam1/latest_snap.jpg WITHOUT local here!
With regards to the URL part, I have the same thing. I want to show some pictures in lovelace and link it to the app, no go. So when I click the Companion app pop-up it links me to that specific image in the app. Only way this works is putting it in the www folder structure, but I like to avoid that since that is publicly accessible in theory.
You can look-up the auth token in the media browser (find the pic, right click it and open in new browser window) but I presume the auth token will expire sooner or later and that will lock you out of home assistant since it will ban your IP after x unsuccessful auth attempts.
it looks like lovelace / picture card / mobile app and the media service don’t play nice together yet.