As mentioned, anything by Roger Dean, especially "Fragile" and "Close To the Edge" by Yes.

Hipgnosis created a very cool cover for "Elegy" by The Nice and for those of you who are old enough, please look at the cover of "Mom's Apple Pie" (same group),  I can't believe this was ever printed! 

I have Mom's Apple Pie ..it was a different world at that time.

10CC - The Original Soundtrack

Emerson, Lake & Palmer - The Best Of"

Pink Floyd - "Ummagumma"

@rayd and @slaw: That original Mom's Apple Pie cover was a result of no one at the record company noticing the female genitalia hidden inside the pie. No pun intended.