Art is, obviously, primarily a 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 This is generally not an issue for non-commercial unpublished work, but it is something to be aware if further academic research is on your horizon.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
Google Arts & Culture
Explore collections from around the world with Google Arts & Culture, created by Google Cultural Institute. Say what you will
about Google, this is a nicely curated site that makes a lot of great art that much more accessible.