Mine was from a feminist-pro porn booklet published in 1986. I found it in a leftist bookshop in 1989 and bought it for $10, it was a major wanking focus for several years. My favorite picture was a b&w of a woman with closed eyes, and a substantial wallop of cum dripping out of her closed lips, next to the dick which was its obvious source as it was strongly gripped by its owner with a pool of cum under the head on the top finger. Her perfectly passive, accepting post just mesmerized me. For years I wondered where it was from, and just a few years ago someone on this site pointed out it was a frame from the 1979 porn film "China Sisters." I've attached below the pictures I had, as well as an original frame & gif from this scene in the film. As usual the film had nothing special to commend it, and in fact I don't think the moving scene would have grabbed me as much as that one picture did in b&w. Somehow in isolation it helped me imagine a better scene than the one it was actually part of, where I wanted her to stay as still as possible as the cum kept dripping and the dick kept wanking.