Need some extra help narrowing it down? Some popular image collections include:
Artstor - includes more than 2.5 million high resolution images from museums, galleries and collections around the world. Artstor contains images of art, architecture and graphics, as well as photography, costumes and historical crafts
CSIRO ScienceImage - a free image library specialising in science and nature images
MedPics - a free open-access database of medical images, teaching cases, and clinical topics
Oxford Art Online - contains images of artworks, structures, plans, and artist signatures
Google Cultural Institute - Google has partnered with hundreds of museums, cultural institutions and archives to host the world's cultural treasures online. You can find artworks, photographs, sculptures and more.