The Google Art Project provides files of artworks that allow the user to get up close and see details. For example, this link leads to a John Singer Sargent watercolor:
There is a small box on the right in which you can zoom in/out (- and +). You can also click and hold in that mini-image to move around the painting. Thus, you can really zoom in and see his painterly brushstrokes.
You can look up other artists, too, of course.