Quickly noting some resources I've come across for visual material on games - please reply with others! Always check the license and attribute the source.

14,000+ CRPG screenshots: https://www.flickr.com/photos/crpgbook/albums

Mobile game screenshots: http://gamesinspiration.com/

More mobile game screenshots: http://mobilegamepatterns.com/

Images of board games: https://boardgamegeek.com/images/

The Play Generated Map and Document Archive: http://www.plagmada.org/Welcome.html

Strong Museum of Play collections: http://www.museumofplay.org/online-collections/