Prefixes for images in url?

Question

Images

Hello. I'm not even sure if I write the essence of the question correctly.
I often see links to images of this kind:
1) site.ru/image.png - full size
2) site.ru/image.png_size_100_100 - size 100x100 pixels
Why this is necessary is clear, but how could all this be done? Your help is very much needed.

Answer 1

for example, you determine from which device your resource is viewed (css media queries). if it's a phone, then connect a special css and give the user thumbnails, thus optimizing the page loading speed.