My idea for wanting PNG encoding is that they have lossless compression, and that the other common format that preserves details is bitmap, which has no lossless compression. At least PNG might compress photos somewhat. The web is plagued with images that have been resaved over and over that have JPEG degradation. I did not know JPG-XL had a lossless format so that's promising imo. But JPG-XL isn't supported in many browsers.
Images, by their nature, are far more fluid, which is why a FFT-based compression algorithm, like the one in jpegs, makes sense for photos.