I’ve always been fascinated by this cover art, the artist is simply listed as ‘Corrigan’. On the back of the Castor LP it has a passage about the cover, possibly written by Corrigan (?):
“The illustration depicts Jimmy Castor in a double role: as a Gemini and Cancer personality. Gemini’s aspects are described as egocentric, intellectual and airy. Castor’s egocentric aspect is affirmed by his unconcealed face; his intellect is symbolized by the tube running from a fitting on his temple to a fitting in the musical tree. His mind drives his music. A second tube runs on to a small cluster of clouds hovering near Castor’s head, symbolizing his airy and mutable nature. Castor’s Cancer aspects are symbolized on the right by the slum tenement which rises out of wasteland. Castor is directly above a large opening in a wall containing his heart; this serves to stress Cancer’s intuitive qualities. Castor’s face is partially concealed to correspond to the introverted nature of Cancer personalities. The tube which runs from his heart is attached to an armored egg; spilling out of the egg are three elements associated with Cancer: the moon, water and food. The prehistoric, “Troglodyte”-like figures are the end products of the intellectual, emotional and astrological input of Castor from above.”

After a bit of help from fellow facebook-ers, it seems the man responsible was Dennis Corrigan, a surrealist American illustrator who only has one other record sleeve listed on Discogs (The New York Rock & Rock Ensemble, front and back covers below).

Still with us and now teaching at Marywood University in Pennsylvania, he has work in various art institute collections and illustrated for the likes of The New York Times, Random House, Pan American Airways and more. He’s had several books of his work published, ‘The Amusement Park’ from 1982 and ‘True Love Knows No Boundaries’ from just last year. You can find more about his weird world at his website, Dennis Corrigan Artist.