WikiPaintings is a new online encyclopedia of painting. It’s pretty well stocked, as well as having a nice clean interface. It mixes public domain works with copyright works, and sadly there’s no way to filter to get “just the public domain works, above 1800px in size” in search results.

Although you can go in via Google Images, with the following search:

site:http://www.wikipaintings.org/en/ “artwork is in the public domain”

This gets about 90,000 results. You can then refine by size in pixels.