Browser resize without the image getting pixly

What i want to know is if someone knows how every site i’m looking at makes their sites rezise without their images gets pixly?

Sites like http://www.hm.com/us/fashionagainstaids#artists/1

Their images looks sharp nomather how big my browser window is… when i try to do the same for my site, it looks like this: http://www.digitalfar.se/tmp/test.html

Not so good :frowning:

Does anyone know how to to it right?