Wild, Fanciful, and Often Trippy

I don't think it's much of an exaggeration to suggest that the covers of science fiction and fantasy novels have gotten much less imaginative over the years. By the mid-1980s, the writing was already on the wall and the wild, fanciful, and often trippy covers that simultaneously attracted and frightened me as a kid were on the way out, to be replaced by an endless parade of Michael Whelan, Darrell K. Sweet, and their imitators. This is no knock against Whelan, who's a great artist, but there is a certain predictability to even his best work that I frequently find disappointing. Come to think of it, predictability might well be the defining characteristic of post-1970s SF and fantasy art, itself a reflection of the mainstreaming and commodification of these genres. (Cue my inevitable dig at much of the oeuvre of Larry Elmore.)

Science fiction and fantasy were still (relatively) fringe interests in the 1960s and '70s and the artwork from the period reflects that. Take a look at these three different covers to the paperback releases of Michael Moorcock's The Stealer of Souls, starting with the Lancer edition of 1967:

I have a certain fondness for this cover, because my local public library still had a copy of the book on one of its spinner racks, where I first saw it. Jack Gaughan, best known for his work on the unauthorized US printings of The Lords of the Rings, is the artist of this piece, depicting Elric in battle against the reptilian demon Quaolnargn, summoned by Theleb K'aarna as part of a plan to separate the Melnibonéan from Stormbringer, while the spectral visage of (I assume) Yishana watches. 
The 1968 Mayflower edition took a completely different tack: Bob Haberfield, who'd go on to do the covers of many more Elric novels, is responsible for this one, which is a terrific example of the kinds of covers I remember well from my youth. Unlike Gaughan's Lancer cover, this one has no obvious connection to anything that occurs in the novelette. That's pretty much par for the course in the late '60s and throughout the 1970s.
Finally, there's another Lancer edition, this time from 1973. This piece is by Jeff Jones, who had an extensive career as a comics illustrator and I think that shows in the cover. I'm not entirely sure what it depicts, though my guess is that it might be the naval assault on Imrryr from The Dreaming City, with the monster being a Melnibonéan dragon. In any case, it's a very dynamic piece that grabs the attention, which is exactly what the covers of science fiction and fantasy covers used to do.