An example: louvre.fr:CA+1814 is a Middle Geometric (800–750 BC) Jug from Attica. The photographer of the image on the linked page is Marie-Lan Nguyen and many more examples of her work can be seen on Wikimedia Commons.
The page for her image of louvre.fr:CA+1814 has the following text: "I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide."
This is fantastic and I'm happy to recognize Ms. Nguyen as a personal hero.
I did a little bit of looking on the Louvre website but didn't see any publicly available information about in-gallery photography. In contrast, the Metropolitan Museum of Art in New York has an easy-to-find policy. It reads in part:
Still photography is permitted for private, noncommercial use only in the Museum's galleries devoted to the permanent collection. Photographs cannot be published, sold, reproduced, transferred, distributed, or otherwise commercially exploited in any manner whatsoever.
I am not a lawyer but it's not clear that the availability of this photo on flickr does not violate these terms. It's not commercial use but it is reproduction and/or distribution. The image is of the Met's Proto-Attic Neck Amphora by the Nettos Painter, one of its great treasures, and I'm particularly pleased to see it because I couldn't find a description of that object on the site. I.e., flickr-user mharrsch (aka Mary Harrsch) is filling a gap in the visual documentation of ancient Mediterranean material culture.
But can she do this? I'd love to know because I'd love to be confident that this and similarly produced images are going to remain available and that I and others really can download, store and reuse them for non-commercial purposes.