Ho to set fixed with for all images I post on WordPress blog?

How could I set up my website so that every picture I posted would be at a fixed width of 600px and then the height would scale proportionally? Thanks in advance!

To make all your posted pictures 600px in width while keeping the original aspect ratio you can add this to Theme Options – Other – Custom CSS or inside style.css of WordPress Child Theme if one is used

img.size-full,img.size-large {
	width: 600px;

This should do the trick.