There's dozens of great l.p. covers.
I love Roger Dean's work and if you want to talk about iconic covers, Roxy Music's early albums have to be on the list.
Personal favourites that jump to mind:
Led Zep's Physical Graffiti but all their albums had heavily designed sleeves and could be deemed iconic.
Pink Floyd were another band who put real effort into their covers; is any cover more iconic than Darkside?
Destroyer by Kiss.
Who's Next.
Queen: A Night at the Opera.
Rush: 2112 & Moving Pictures.
Elton John: Goodbye Yellow Brick Road.
Rainbow: Rising.