Paul Klee in 1911. Image Source: Wiki.
A super new Website, still in beta, WikiPaintings, is a Visual Arts encyclopedia that displays great paintings online. A nice example: all of Paul Klee's works - which spanned expressionism, cubism, and surrealism in the first half of the 20th century - are shown in a WikiPaintings gallery in chronological order. You can scroll through Klee's beautiful gallery here.
Paul Klee, Die Zwitscher-Maschine (The Twittering Machine) (1922). Image Source: Wiki.