You must absolutely check out Getty Museum's online images.

The Getty Museum has made much of their collection available online to view, download and use freely.  If you choose to download the full-resolution images, they're big, as in a dozen or two megabytes.  Fortunately they have this great viewer online to zoom in and out (see screen captures below).

This is one heck of a collection The Getty Museum is putting up, and deserve a lot of credit for making it easily accessible.