@OlivierBégassat If you remember the picture, you might try to search using google images with adding some keywords and restricting to this site.

Perhaps also searching for posts in some specific tag linking to a picture might be reasonable. Like url:"jpg" [algebraic-number-theory] and other variants of this search.

Re: there was another thread in the vein of "most beautiful pictures in math". Did you perhaps mean Examples of unexpected mathematical images?

Of course, it is also possible that the images you're looking for are already gone - like the ones mentioned here: Restoring Bill Thurston's figures in an answer.

Hi @MartinSleziak, I managed to locate a similar thread on MSE (math.stackexchange.com/questions/2749930/…) with a link to Baez's thoughts on the matter (math.ucr.edu/home/baez/roots)