How are progressive images uploaded?

Question

Frontend

So it goes.
I often come across a loading method, when at first we are shown a very poor quality picture, therefore a little better, even better, and then the original (there can be more or less steps).
With Google pictures it seems clear - first a reduced copy is shown, because it is muddy.
But VKontakte does it a little differently. There, poor quality is obtained with the help of a mosaic - the image is blurred in squares:

Is this some kind of software work or are there several processed images?

Answer 1

Probably Progressive JPEG - https://habrahabr.ru/post/165645/