it makes browsing feel tactile and directional, instead of being assaulted by 40+ provocative images, gore, porn, etc at a time

by blurring images until mouseover you fundamentally change the way your brain interacts with an image board
you see the text first, allowing you to parse whether or not a thread is spam, likely using an attention grabbing provocative image to lure you into the thread and waste your time

img {
filter: blur(16px);
transition: filter 0.3s ease;
}

img:hover,
img:focus {
filter: blur(0);
}

paste this into the CSS box in advanced settings, you need 4chanX extension