The images are sourced from museums and libraries with open access policies all over the world. We are currently working on adding sources, locations and other metadata to all the images as well as an API.
Some feedback if you'd like - the infinite scroll is a bad browsing experience when used for huge amount of work: - picture loading starts slowing down when scrolling too much. - when a reach a point I cannot save my browsing process. For example I browsed the Abstract category. There are so many works that I can't possibly see them all in one sitting. Next time when I sit to browse I can't continue from where I have reached.
My use case for your site would be to find good wallpapers for my PC. Apart for the browsing experience it would be nice to be able to filter images based on resolution or aspect ratio so that I can find wallpapers. You mentioned you are making an API so maybe that could solve that.
Art needs to be seen and the best place for me to see is on my desktop wallpaper :)