Pretty much anything by Storm Thorgerson or the Hipgnosis studio... think all of the Pink Floyd and Alan Parsons albums, and a good deal of Led Zeppelin albums plus heaps more.
Hear, hear!
Of the Hipgnosis art, I would have to say my favorite is

Also, 10,000 Days, by tool, some of the inside art is just amazing, not the least of which being

Then there's Moving Pictures, by Rush
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Rolling Thunder, The Grateful Dead
All of Journey's covers.
Dirty Deeds Done Dirt Cheap, AC/DC

Ladies and Gentlemen: The Grateful Dead

Well, somebody's already posted the White Album, how about the Black Album?
(Sorry, can't find any good photos easily)
Speaking of Black albums:

And, of course, all of Ozzy's covers, but this one in particular:

I can go on and on and on until I've posted hundreds of photos. but I'm going to leave bandwidth for everyone else. Also, popcorn calls me.