If you mean where the image files come from, they're hosted at https://s3.amazonaws.com/artful-collection. Using Chrome will just give the xml structure of the bucket, but you could parse and browse yourself with that much.
no, I meant where did you take the images in the first place. I had a thought in the past of making an app that used classic art in some way, and IIRC I found that http://www.artchive.com/ lets you use their art collection for commercial purposes, while the Google Art project https://www.google.com/culturalinstitute/project/art-project doesn't.
I think that even if the original art has no copyright restrictions, the reproductions of it can have some.
Nope, nope, nope. Reproductions can never have their own copyright. Really: No matter how much work someone does in tracking down and scanning public domain art, the "work" in question is still in the public domain. Only authors get copyrights, not "sweat of the brow" labor.
You can get a copyright in any creative changes or additions you make to a public domain artwork--the things that you are the author of.
It is sometimes a violation of Internet norms to fail to give credit to someone who has originally scanned something, but this is not copyright.
