Can you edit your post?
Probably not. HN only allows editing comments for a limited amount of time. In one post I found from a couple of years back, someone said that the edit window is two hours. I think perhaps also that as soon as someone has replied to a comment it will not be possible to edit that comment any longer but I'm not sure about that.
... so evilcorp.org cookies for evilcorp.org images on A.org will not be set for evilcorp.org images on B.org.