Art Cyclopedia keeps a collection of links to different museums and other web pages where art can be viewed online. You can search by artist, title of an piece, or by museum.
Art History Resources provides links to time periods and geographic locations through the history of art.
The Timeline of Art History pairs the Metropolitan Museum of Art's Heilbrunn Timeline of Art History with essays and works of arts with chronologies, telling the story of art and global culture through the Museum's collection.
Google Images lets you search directly for pictures. You can also drag/drop a picture into the search bar to look for more information about it.
Art Assignment is a YouTube channel produced by Complexly and PBS, hosted by curator Sarah Urist Green. They explore an variety of art topics including famous artists, eras, and styles.