Trust me you are seeing it, there are just updated ways to handle improved image loading. Twii for example (one of my projects) when you scroll loads images on demand, it also uses a internal image processor with cache so it will load dynamic the image based on the container size holding the image and if you are using a HIGH DPI display so the images are always the best fit down to the nearest 50px (rounded up).
Facebook, Instagram, and many others have some kind of image loading, the key is process when users are scrolling, and go 300+px below the fold so when the user gets to the image it is ready and done and they do not see it.
Facebook and Instagram actually send a smaller version of the image usually 30x30 version that is scaled up and blurred so the transition is softer and easer if the user sees the image before the full image is loaded.
Here is the blog post on Facebook about this system using GraphQL.