Below: Wes Wilson
Originally coined by The Beach Boys, the word "psychedelic" has heavy connotations in our society to a specific kind of people. These people are open to expanding their perception and investigating the overlap between art and drugs. They are the hippies. And as important to hippies as their freedom of expression was, they loved the arts.
The psychedelic culture of the 1960s brought about a new appreciation for rock-n-roll with breakthrough musicians like The Beatles and Jimi Hendrix. With this new surge in musical renaissance, a need for art to accompany this music arose. Psychedelic posters drew inspiration for their bulging curvilinear style not only from LSD visuals but mostly from Art Nouveau and some Dada.
The most important aspects of a good psychedelic poster are starkly contrasting colors (to make them stand out), elaborate detail, wide curves, and most importantly, bizarre surrealism that makes one feel "dazed."
I personally love psychedelic art and the artists/musicians associated with it. While I am not a hippie, I still greatly appreciate the culture and the aesthetic that these artists were striving for. They were definitely not out of it when they created these beautiful posters that are really well thought out and revolve around complex themes. The artists conveyed an image and attitude for rock stars through a visual medium in a time when computers did not yet exist. They could not make a Myspace page to promote their band. And music was a lot less convenient to listen to than today. You could not just sample any band's music with the click of a button. Bands had to attract their target audience through these psychedelic posters. If you like the poster, it's very possible that you would like the band.
Above: Bonnie MacLean
And yes, psychedelic posters are hung in art galleries.