pixivImageFit.user.js

// ==UserScript==
// @name         Pixiv Image fit
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Makes full image fit to screen
// @author       NO_ob
// @match        https://www.pixiv.net/en/artworks/*
// @icon         https://www.google.com/s2/favicons?domain=pixiv.net
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    var head, style;
    head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = 'div[role=presentation] > div > div > div > img {max-width:100%}';
    head.appendChild(style);
})();