I am not seeing any flickering when I remove the opacity, so I guess we sorted the issue out?
I fixed (1) per:
https://github.com/discourse/discourse/commit/001c593155e8e6a53715cd9485c7c594e4999a23
But I still feel the meat here is that the issue is not properly discoverable and depends on hover.
@Johani maybe do a theme component for styling this now (since you can do pure CSS to get whatever you want) be sure to test this on a very very short image. Then install the component on dev and meta so we can live with it for a bit and decide if we prefer this or not?