Actually it's more serious than just the image. If you add a new comment when not logged in, the whole post with the new comment is entered into the cache and served OK. But an immediate additional comment added is not served because the cache hasn't expired. The comment count is also not updated.

I have tried adding the Nginx proxy cache integrator which adds effectively expires the cache where particular types of cookie are found but a visitor does not (in general) result in a cookie so the image problem at least remains.