Thanks, eishiya. I'm not sure I understand your explanation but I will check it out.

The examples by Ilkke are not entirely what I want to achieve. Ideally, I would like to be able to lay a polar grid on an image so that the first circle in the middle has 12 pixels, the second has 24, then 36, next round 48 etc. But I can work with 6-12-18 and so on too if there's no other option. In any case, the pixels should have more or less the same size (small differences I can live with).

I have seen similar things with dots or halftones instead of pixels. Usually, these start with only 5 dots in the first circle and then 5 more each next round.

https://www.vectorstock.com/royalty-free-vector/abstract-world-map-of-radial-dots-vector-20645503

Another approach is this where you have uninterupted lines in concentric circles or spirals. This actually comes close to what I need but instead of lines I want pixels in different colors or different gray shades. I'm referring to the portrait of Audrey Hepburn by this Etsy seller:

https://www.etsy.com/nl/shop/LinesLab?ref=l2-shopheader-name

And then there's this example with radial pixelation except that there are more than one colors in each "pixel" and I want just one.

https://kentuckyartspeaks.wordpress.com/2012/03/03/an-informative-book-about-figurative-art-that-may/