Another great part of the poster/portfolio/fine art era in comics was the MARVEL COMICS INDEX series by James Olshevsky.
Aside from indexing the Marvel era in extraordinary detail up to that point (1975-1982), These books also sported some very poster-worthy covers by some of the best artists of the period.
Here are a few:
INDEX 2: CONAN (Tim Conrad cover )
INDEX 4: FF (Steranko cover)
INDEX 5: THOR (Tim Conrad cover )
INDEX 6: DR STRANGE ( Tim Conrad cover )
INDEX 7-A: HULK ( Ken Steacy cover )
INDEX 7-B: SUBMARINER ( Everett cover )
INDEX 8-A: CAPTAIN AMERICA ( Steranko cover )
INDEX 8-B: IRON MAN ( Ken Steacy cover )
INDEX 9-A: X-MEN ( Brent Anderson cover )
http://fullsize.9.A.X-MEN.GIFINDEX 9-B: ( Frank Miller/ Klaus Janson cover )
http://fullsize.9.B.DAREDEVL.GIFAlthough far from the cheapest I've seen these books, the images of all of them, along with their original release-dates, are listed at this link:
http://www.mycomicshop.com/megastore/All&til=1448