This is a relatively famous painting. This is in the public domain because it was copyrighted before 1923. Anything copyrighted before then is now in the public domain. That's very good because you can get things for free now that could have earlier sold for who knows how much money and you can use them however you like. I got this image of a painting by Georgia O'Keefe from
cgfa.floridaimaging.com. Not everything there is in the public domain, though, so you'll have to pay attention to when the images where copyrighted.