It is no secret that I love paintings and art galleries in general. The National Gallery in the US has opened its digital vault to allow access to thousands of high resolution images of its work here.
From the website:
NGA Images is a repository of digital images of the collections of the National Gallery of Art. On this website you can search, browse, share, and download images. A standards-based reproduction guide and a help section provide advice for both novices and experts. More than 25,000 open access digital images up to 3000 pixels each are available free of charge for download and use. NGA Images is designed to facilitate learning, enrichment, enjoyment, and exploration.