Make Kijiji use higher-resolution listing images.

embiggen-kijiji.user.js

// ==UserScript==
// @name        Embiggen Kijiji
// @description Make Kijiji use higher-resolution listing images.
// @namespace   ca.seenet.userscripts
// @author      Samuel Coleman
// @version     2017-01-18
// @match       http://www.kijiji.ca/*
// @grant       none
// ==/UserScript==

/* The lightbox images are initialized via an inline function call toward the
 * bottom of the page. We'll monkeypatch the function to filter the input. */
Kj.Vip.initImageLightBoxDisplayAdBase = Kj.Vip.initImageLightBoxDisplayAd;
Kj.Vip.initImageLightBoxDisplayAd = function (options) {
    options.items.forEach(function (item) {
        /* All Kijiji/eBay images have a "$_dd.JPG" parameter on the end of them
         * (where "dd" is a number from -1-140) which controls the size/crop.
         * "27" is the default large size (up to 640x480); "57" is the largest
         * (I've seen 1600x1200 on eBay, although it's usually at most 800x800
         * on Kijiji). */
        item.src = item.src.replace(/\/\$_27.JPG$/, '/$_57.JPG');
    });
    return Kj.Vip.initImageLightBoxDisplayAdBase(options);
}

var style = document.createElement('style');
style.innerHTML = `
/* Prevent the larger images from unnecessarily lengthening the page. */
div.mfp-content {
  top: 30%;
}

/* The lightbox <img> tags are styled to limit their size. Override it. */
img.mfp-img {
  max-height: 100%;
  max-width: none;
}
`;
document.body.appendChild(style);