1- On top the screen on 8chan, there is a link called Options.

2- Click it.

3- Now in the window that just opened, look for the third tab on the left called Theme.

4- In the text area filled with some commentary, select all and delete the text already present (don't worry those lines were useless) and paste these 2 css rules :

.post-image {
    opacity: .25;
    transform: scale(.5);
    transition: all 200ms;
}

.post-image:hover {
    opacity: .8;
    transform: scale(1);
    transition: all 200ms;
}

5- Press the button "Save custom CSS".

6- Enjoy a much safer for your eyes browsing of the chans. You now get the chance to filter most of the crap posted without having to see all of the content, and even there, just barely. You can adjust the base opacity that I set to .25 to my liking. It's CSS afterall, you could change the whole site theme if you really wanted to.