Some free online collections:
  • Smithsonian collection at a search for Paintings before 1800. Drill down further by "topic" e.g. "Mountains" or "scholar"
** collections.si.edu/search/results.htm?q=&fq=online_visual_material%3Atrue&fq=online_media_type%3A%22Images%22&fq=culture%3A%22Chinese%22&fq=object_type%3A%22Paintings%22&view=list&date.slider=300s%2C1800s
Unclear legality:
Figure 1. Early Autumn by Qian Xuan (13th century) Source.
Ciro Santilli's favorite. A mixture of painting and book.
Figure 1. Source.
Figure 1. Source.