Artstor offers many CC0-tagged files in the Digital Library. CC0 is the "no copyright reserved" option in the Creative Commons Toolkit.
To search for CC0 content, use the keyword search box and enter "creative commons zero".
Or, use the Advanced Search box and use the same keywords, along with further selections, to narrow your results.
The metadata contains links to the contributor's page, where you can download high-resolution files.