Set Post Thumbnails in Percents

This is not possible, images must have real absolute sizes 🙂

A post thumbnail is essentially a different sized version of the image. While you can calculate how much for example 70% of 1024 is, and genereate a ~700px wide image, you cannot do this for each of your visitors. It would kill your host’s CPU…

The closest thing you can do is to browser-resize images to fit their container:

img{
  max-width: 100%;
  height: auto;
}