If you're extremely interested in art, you presumably already know about Olga's Gallery, one of the web's largest and most accessible collections. I've been going there for years whenever I felt the need, as I sometimes do, to look at paintings, or whenever I need to follow up a reference to a particular work of art. You can search by artist, country, movement, or name.
Clicking this link will take you to the gallery for Nicolas Poussin (1594-1665), one of my favorite painters. Clicking this one will take you to Nicolas Poussin's dark and enigmatic scene from Christ's last supper (The Eucharist, painted in 1647), my very favorite. I love the expression on young John's (I assume it is young John's) face.