JSFiddle - React, Tailwind, and code Playground

HTML

<img alt="alttext!" src="http://cdn.sstatic.net/stackoverflow/img/logo-10m.svg"/>
<div style="background-image : url('http://cdn.sstatic.net/stackoverflow/img/favicon.ico');">xyz</div>
<br/>

JavaScript 1.7

// ==UserScript==
// @run-at document-start
// ==/UserScript==
"use strict";

/* part one: <img> elements */

(new MutationObserver(function(Records, Obs) {
	for (let R of Records) {/* examine each mutation record: */
    	/* if the record specifies an attribute mutation… */
		if (
        	R.attributeName === "src" &&
			(R.target instanceof Element) && /* this check might be necessary */
            R.target.tagName.toLowerCase() === "img" &&
            R.target.getAttribute("src") !== "" /* avoid infinite loop */
        ) {
        	R.target.setAttribute("src", "");
        };

		/* if the record specifies a sub-element mutation… */
		for (let N of R.addedNodes) {
            if (
				(N instanceof Element) && /* this check might be necessary */
                N.tagName.toLowerCase() === "img" &&
                N.getAttribute("src") !== "" /* avoid infinite loop */
            ) {
                N.setAttribute("src", "");
            };
        };
	};
})).observe(document, {
	/* changes wot we listen for */
	childList : true,
    subtree : true,
    attributes : true
});

/* part two: background-image styles */

let check_for_head_elem = function(_, Obs) {
	if (!document.head) {return;};
	Obs.disconnect();

    /* apply our style */
    let Style = document.createElement("style");
    document.head.appendChild(Style);
    Style.sheet.insertRule("* {background-image : none !important;}", 0);
};

let check_for_root_elem = function(_, Obs) {
	if (!document.documentElement) {return;};
	Obs.disconnect();

	/* observe until the <head> element is added */
	Obs = new MutationObserver(check_for_head_elem)
	Obs.observe(document.documentElement, {childList : true});
	check_for_head_elem(null, Obs); /* check here because it might exist already */
};

{/* observe until the <html> element is added */
	let Obs = new MutationObserver(check_for_root_elem);
	Obs.observe(document, {childList : true});
	check_for_root_elem(null, Obs); /* check here because it might exist...