The Metropolitan Museum of Art in New York has released thousands of its works of art online to the public domain, in a new ‘Open Access’ initiative, in collaboration with Creative Commons (a non-profit organisation that aims to promote the legal sharing and distribution of information, ideas, and images through its free and easy-to-use copyright licenses, in this case Creative Commons Zero).
This means all the works released under this licence are available for unrestricted use – you can copy and modify the work, even for commercial purposes, all without asking permission.
You can access these images through their search page, making sure you tick the ‘Public Domain Artworks’ box on the left hand side. To give you an idea of just how many there is , and the outstanding quality, I did a search under ‘Public Domain’ and ‘Relief Printing’, which brought up over 9000 images, some of which you can see below.

South Wind, Clear Sky (Gaifū kaisei), also known as Red Fuji, from the series Thirty-six Views of Mount Fuji (Fugaku sanjūrokkei) by Katsushika Hokusai ca. 1830–32