Art in the Christian Tradition
The Art in the Christian Tradition (ACT) database is a freely available, regularly updated visual image internet resource. All images may be used for educational and/or religious non-profit purposes.
http://diglib.library.vanderbilt.edu/act-search.pl
SBL's website Bible Odyssey has many excellent resources including an image gallery.
Artwork is indexed according to Biblical/Liturgical subjects and also listed by liturgical year.