The collage has long been a part of the rock 'n' roll album-art style book — most memorably punk's political-statement by juxtaposition — but also in classic rock like the Beatles' iconic "Sgt. Pepper's Lonely Hearts Club Band" cover. In fact, the first true album cover, ever, was a rather striking collage, of sorts. It seams the style has come back in vogue, not that it ever went away, but I'm seeing more and more appear on both pop and indie rock covers — not all of them indebted to punk.
Vintage magazines provide the best and most affective source material for album-cover collages, conveying a perverted, commercialized version of past lives and trends. It provides apt ammo for one traditional target of alternative music — consumerism. Or sometimes it just creates a skewed nostalgia affect. With the Internet, creepy old advertisements or any other printed ephemera is almost too easy to gather for art purposes.
I had some fun with this style myself here and here. (anyone in need of an album cover, my services run as cheap as the quality of my work.)
Oasis - "Dig Out Your Soul" (2008)
Thermals - "Now We Can See" (2009)
Thermals - "The Body the Blood the Machine" (2006)
Heartless Bastards - "The Mountain" (2008)
Panda Bear - Person Pitch (2007)
N.A.S.A. - "The Spirit of Apollo" (2009)
(editors note: this album sucks, but it might be my favorite cover on the list)
Ben Harper and the Relentless 7 - White Lies for Dark Times (2009)