If you are looking for classic or famous art depicting Biblical or Christian texts or themes, here are some places to start. Images from these sites are made available by the galleries for public, non-profit use free of charge.
Web Gallery of Art
Rijks Museum, Amsterdam
Art Institute of Chicago
The Louvre – some works available for educational use – see Terms and Conditions 4.1 when you download an image
Vanderbilt University – Divinity Library
Metropolitan Museum of Art
Smithsonian Open Access