Ok. I think it is fair use.
I always link back to the original post. I always look for specific copyright information about the picture. If anyone complains about a specific image I am happy to delete them. They are not necessary to my argument in any case. They are basically eye candy, or things it would be nice to be able to see in conjunction with what is being said.
As for book covers they are works for hire for their publishers and I think the publishers would see this as free advertising.
If they are modern things I either look for the copyright info, or have at times gotten permission of the artist or photographer to post the work.
But most of what I post are about ancient things that no one cares about or my own Wordle creations.
I think you folks are going overboard, but if Quora decides that I should take out the photographs I have placed in my post, then I will be happy to remove them.
Coda: I have been asking for permission and every publisher so far as granted permission. Even for in one case for a long extract that violated their stated policy. Several said that they did not expect to be asked for permission in cases like this. But for Book covers basically they say that any time their books are featured that is good unless it is somewhere that would detract from the book. Several of them did want the their website associated with the book, so I plan to do that in the future. But I think this issue is settled. Publishing book covers is OK and my original interpretation of the rules of fair use was correct with regard to book covers.