HAL9000 from Italy has been posting ultra high resolution photos of famous paintings. In 2007 they posted a 16 billion pixel photo of Leonardo da Vinci's Last Supper. They've since increased the pixel count of the photos.
Their more recent image of Botticelli's La Primivera consists of 28 billion pixels, about 3,000 times the resolution of a consumer digital camera. The pixel density (pixels per inch, or ppi) has also increased, from 580 to 1,500ppi (magazine and book printing are typically 300ppi).
In contrast to the "gallery view" afforded by the online Monet exhibit (in which you can see individual brushstrokes wonderfully), these images are more like a "conservator's view", allowing you to zoom in to a level as if observed under a magnifying lens.
You need to be patient with the image as it loads, but once loaded, the interface is remarkably responsive as you zoom. The images are watermarked, but that's a small quibble considering what they are offering, and you can work around the watermarks by altering the magnification level and scrolling a bit.