All are "memorable" album covers, but Abbey Road (like Pepper) is one that when you see it or see a spoof of it, you know exactly what it's related to.
I think in order to be deserving of the title 'Most Recognizable Record Cover', you first need to be able to take off the artists name and album title to see if it's still recognizable. Secondly, show it to about 100 people ranging in age from say, 15-65. To that end I think 2 albums fit the bill.
Dark Side Of The Moon Sticky Fingers
Of course there's Abbey Road and Sgt Pepper. But they have the band on the cover as a clue, so I'm not counting them.
I just want you to reassure him - talk to him, make him see the error of his ways. Then I'll hit him.