Edit: 14/11/2023 I downgrade my 5 previous stars review. Unfortunately this addon does not work anymore on Google Arts all the the time . Something about too many requests on the server.

The author has created another tool called Dezoomify - rs that works and you can download the images. You have to install it on your computer and Windows Defender warns you that it may be insecure. I passed Norton and it seemed perfectly fine.

I hope this extensions gets fixed in the future or a new one comes along with the same functionality and success rate this one had in the past.
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Excellent app that I use to capture the full resolution images that are so difficult without this kind of extensions. Just sent a little donation to support such an excellent tool