Web dev here: They're lazy-loading the images to give a perceived faster load time. You take a super low quality (often blurry so it doesn't just look low quality) placeholder and swap it in for a proper image as needed.

This is overall a slower loadtime and a bit wasteful (sending useless junk over the wire) but appears to load faster for end users since the time-to-first-render is quicker. Of course, lazy-loading the images requires Javascript to be enabled to detect when the page has finished loading and it is now "safe" to load the higher quality images.