Are there any guidelines for thumbnail image sizes?

Question

Standardization

1. As I understand it, browsers are well strained when images are stretched back and forth and it would probably be useful to cut the preview close to the size that needs to be shown on the page, but today almost all sites are responsive and block sizes are in percent. Perhaps you should think about mobile sizes first of all? Or is this all a misconception?
2. Before today, we somehow cut pictures: 120x90, 320x240, 800x600 and stuff like that. Is this relevant today? Maybe it's worth having only two files: 1 - original or close to it (max. 2048x2048 ?), 2 - large preview (max. 400x400 ?), and then adjust the size with CSS?
3. I tried google on this topic, but it tells about recommendations for VK, facebook, etc. It turns out that they have some recommendations for pictures, but there are no general recommendations for previews for sites?

Answer 1

do as in (2) , then Google Page Speed ​​will tell you everything it thinks about the site - correct it to the required dimensions