Browse one of the image databases to locate artworks from a specific time period, subject, style, or artist.
Start with Artstor. Ex: revolution
Browse Google Arts Project for ideas of artworks by movement, time period, theme, etc.
2. FIND BACKGROUND INFORMATION
Browse online or print sources to identify specific artists/works of art from different centuries. There are several reference books that can give you a good start, located by the Humanities Help Desk on level 5.
Once you identify an artist, search the name in Oxford Art Online. Ex: Mary Cassatt