Thank you game master for the high res picture !!!

https://twitter.com/coin_artist/status/938483706607611907

This new tweet give us many information about the puzzle. We now have the answer to the following questions (as I see it).

# 1 The private key is only in the painting or also in the digital file ? There is nothing to extract from the digital file, all the information needed are inside the painting (because we have the .jpg and .tif version of the file). I spent time looking for the DCT coefficients histogram in the jpeg file which is useless, there was nothing strange but I wasn't 100% sure, now I have the answer.

# 2 I had the feeling that some text was hidden in the painting, for example, bottom right of the picture, in the jpeg file you can see a "2" in the Grec symbol. I also had the feeling that some text was hidden in a ray of light coming from the keyhole. Thanks to the .tif file I now know that there is no such text in the picture. The "2" is just a JPEG artifact and the text in the ray of light are just brush stroke. So to answer the question one person had on twitter, I don't think there were details we couldn't see in the .jpg file, just things we could overthink.