I added a few nice covers to my Gay / Lesbian Pulp Fiction Cover Gallery.
First this great Robert Bonfils skin-diving cover (I love the frame of undulating seaweed and the shorts tanline): Venus of Lesbos by Steve Bell (Newsstand Library 1961).
and then Slim by William Wister Haines (Bantam, 1959). The text of this book isn't explicitly gay, but come on...
Not sure who the artist was.
Slim was made into the 1937 film Starring Henry Fonda.
Lastly Paul Rader's fantastic cover for Her Private Hell (Midwood, 1963), featuring a really innovative use of side-boob.