Art museums often mount high-resolution images from their collections with reliable metadata and downloading rights on their websites. Sometimes these images do not appear in other places because they are in databases or because of naming differences.
If you know the name of the museum, start there. If not, often searching the country and “art museum” will bring up a core list.
Among the best are:
Several countries host combined collection of the art work from their country.
Historic (“monuments of art”) Collections