You can use these high-res pictures on your personal and commercial projects for free. All pictures on the website snapographic.com by Thomas Mühl and free of Copyright Restrictions (CC0 1.0 Universell) . A backlink is not necessary, but it would be nice.
NGA Images is a repository of digital images of the collections of the National Gallery of Art. The National Gallery of Art implements an open access policy for digital images of works of art that the Gallery believes to be in the public domain. Images of these works are now available free of charge for any use, commercial or non-commercial. Users do not need to contact the Gallery for authorization to use these images.