At the turn of the 19th century in the United States, the dime novel gave way to the pulp magazine as the standard popular literary entertainment. This was the era of the scientific romance, and H. Rider Haggard's colonialist adventures of Allan Quartermain gave way to the sword-and-planet adventures of Edgar Rice Burroughs, whose hero John Carter of Mars and Tarzan of the Apes first appeared in 1912 in the pages of the All-Story Magazine.
Covers helped sell the pulps; while the interiors were cheap paper with black-and-white illustrations or poor photographic prints, the covers were vibrant, full-colored affairs. Artists were churning out luscious princesses, curvaceous demons, and bound damsels in distress every month during the 1920s and 30s; and even the occasional nude figure advertised the adventures of Robert E. Howard's Conan the Cimmerian or C. L. Moore's Jirel of Joiry in Weird Tales. This trend was taken to an extreme in the Spicy pulps, which catered directly to an adult market, though their fiction was not fantastic, and even the science fiction and fantasy pulps like Strange Tales of Mystery and Terror, Amazing Stories, Wonder Stories, and Unknown Worlds had limitations on how far they would suggest sex in their pages.
Hardback publication of fantasy fiction was a dream of the pulps that was seldom realized. You can definitely see the pulp influence on the dust jackets of the first edition John Carter and Tarzan hardbacks, but fantasy novels that didn't come from the pulps tended to be more sedate and literary - Lord Dunsany's first tales of Pegāna in 1905 were illustrated inside with gorgeous fantasy art by Sidney Sime, but the covers were very plain; E. R. Eddison's The Worm Ouroboros (1922) too is not focused on the masculine or feminine form.
The Hobbit (1937) by J. R. R. Tolkien, published the year after Robert E. Howard's death, was classed as juvenile fiction; it stars no masculine heroes or beautiful women a la Howard or Eddison, and Tolkien designed the dustjacket to illustrate his Middle Earth. The works of Howard and Tolkien represented two different schools of fantasy and two different markets. The dust jacket, as packaging and advertising, was a part of that marketing strategy but also a direct reflection of the difference in how the different authors approached their fiction - but not all writers would get a say in how their fiction was presented, artistically. Howard in particular sometimes complained about the artists used for his Conan serials in Weird Tales, but he also included scenes with nude women in his stories explicitly to compete for a chance at a cover illustration.
The pulps waned in the 1940s, due to paper shortages and the rise of the paperback novel, and publishing attitudes shifted. "Pulp novels" were generally non-fantasy, but featured evocative covers and thrilling titles, designed to titillate and draw the reader in, promising sex and violence. Science fiction paperbacks were less daring, more experimental in general, but also understood and appreciated the benefit of a flashy cover to catch the reader's eye, and the covers to works like Avon's paperback editions of Merrit's Seven Footsteps To Satan (1955) definitely echoed the pulp roots, with the occasional topless woman, bound by her hands.
Robert E. Howard and H. P. Lovecraft, leading lights of Weird Tales, saw hardback publication beginning in the late 1930s from Arkham House and in the 1950s from Gnome Press. The art style for these hardbacks was sometimes weird, but not focused on sex or violence; they lacked the dynamic, colorful covers of the pulps, being much more serious and sedate, lot of solid colors. 50s fantasy hardbacks like Poul Anderson's The Broken Sword (1954), Tolkien's The Lord of the Rings (1954-1955), and T. H. White's The Once and Future King (1958) were not explicitly juvenile works, with Anderson in particular echoing the American style of fantasy more than the British, but the covers were very conservative.
In the 1960s, these hardbacks finally came up for paperback publication - and here, market forces shifted. The Lord of the Rings paperbacks, in the ACE unauthorized and then the official paperback editions, were published in 1965 and became a sensation among college crowds, but the covers continued to reflect the style: very sedate, very conservative. Older pulp works were also being reprinted, and these were not sedate: Robert E. Howard's Conan stories began to be republished by Lancer books in 1966 with terrific covers by the now legendary Frank Frazetta and Boris Vallejo...and inspired a wave of imitators.
The paperback fantasy deluge of the 1970s and 80s had those two major influences: Tolkien's fantasy, sometimes called "epic fantasy," and Howard's fantasy, sometimes called sword & sorcery or heroic fantasy. There were other media influenced by these sources as well - comic books (Conan the Barbarian would be published by Marvel in 1970), animation (Richard Corben's Den debuted in Neverwhere in 1968, and eventually the Heavy Metal film in 1981), Dungeons & Dragons (1976) roleplaying game, films like Conan the Barbarian (1982) starring Arnold Schwarzenegger, and heavy metal and black metal bands like Venom (1979) and Bathory (1984) all helped to promulgate that imagery in other media.
It's important to break down that what we think of as the "cheesecake/beefcake" covers of living masters like Julie Bell, Sanjulian, Richard Corben, and Esteban Maroto really started out with Tarzan - the almost-nude, muscular, athletic hero of Burrough's original serials (then hardback novels, then films, then paperbacks, etc.), and Howard's gritty, realistic, violent fantasy fiction which was realized by Frank Frazetta as a muscled, imposing, often scantily-clad barbarian - sometimes depicted with equally scantily clad women, a borrowing from pulp covers. These themes were continued and expanded upon by subsequent artists and illustrators, and they continued in large part because they sold - which even in the 1980s became a point of parody; the Dec 1982 cover of Mag Magazine "Conan Cuts the Baloney" deliberately mocks Frank Frazetta's style - although aimed more at the 1981 film than anything else.