Artstor Image Digital Library [Rutgers restricted] has over 2 million images in the areas of art, architecture, the humanities, and social sciences. Registering for a personal account will allow you to download images, save images into groups, share your groups.
-- Suggested keywords: erotica, sexual intercourse
Brooklyn Museum Shunga (春画 / erotic pictures) Collection. Shunga is a type of Japanese erotic art executed similarly to ukiyo-e, often in woodblock print style. The Japanese word Shunga means "spring image," and spring is a common euphemism for sex.
Museum of Fine Arts Boston - MFA Images Erotica showcases erotica and sexually explicit images of art in the museum collections. The provenance is mostly Japanese but includes works from other parts of the world.
Obsession: Nudes by Klimt, Shiele and Picasso is a selection from the Scofield Thayer Collection of the Metropolitan Museum of Art in New York. It includes erotic and evocative water color paintings by artists Gustav Klimt, Egon Schiele and Pablo Picasso.
Wellcome Collection (London, England). This link presents the results of the keyword search using "erotica." It includes images of different types of art work depicting sexual intercourse and erotica. Wellcome Collection is a "free museum and library that aims to challenge how we all think and feel about health."