Search by artist, artistic movements, medium or places. High quality images. Clicking on image provides you with citation info.
Use Creative Commons search to find images that have been licensed for re-use. Read the Rights section to see if you can use the image. Bet bet: search flickr for images of art, architecture, cityscapes, etc. Be sure to site the creator and give credit in a caption.
Many museums have great websites for their collections -- try a Google search to find a particular museum, or get started with the ones we've listed below.
Here's a list from Wikipedia of the most visited art museums in the world. Each entry links to that museum's Wikipedia page. The official website for the museums can often be found at the bottom of its entry, under "External links."
General collections
By region
Asia
France
Italy
With your Boston Public Library card, you can access Grove Art Online. This resource contains the full searchable text and images of Grove Art Online, The Oxford Companion to Western Art, Encyclopedia of Aesthetics, and The Concise Oxford Dictionary of Art Terms.