this is actually an original feature of jpeg.

back in the day of dialup internet you could embed a smaller thumbnail in the exif data of the jpeg that would load first, faster.

this would allow your website to be “populated” with content but at a lower quality while the full image continued to load in the background.

in the early internet these were even used sometimes to create single play “gifs” that were of a superior quality that gif just couldn’t render.