Art is, obviously, a primarily visual medium. The following are some resources to consult when looking for a particular image, or browsing by style, period or artist. Many images are in the public domain, but some, particularly those in museums, can be subject to copyright and other licensing restrictions. This is generally not an issue for non-commercial unpublished work, but it is something to be aware if further academic research is on your horizon.
Many museums have digital collections accessible online. See the section entitled Museum Resources for further information