A lot of art falls in the public domain. Now the Metropolitan Museum of Art has made it easier for you to use some of it. (And by “some” I mean “quite a lot”.)
For example, a high-resolution photo of Van Gogh’s painting Sunflowers is available for me to use free of charge. You too.
And if Munch’s The Scream is more your cup of tea, well, they’ve got you covered there too.
Have a look for yourself. If one of the 400,000+ pieces of art in their catalogue strikes your fancy, check for the Public Domain icon (a little 0 in a circle) and the download link. It appears that the public-domain content comprises 375,000 images, which, I think you’ll agree, is a lot.