Script to stop auto play of gifs in the chat.

gifmask.user.js

// ==UserScript==
// @name          GIF Mask
// @author        Robert Lemon
// @version       0.0.7.1
// @namespace     
// @description   Replaces gifs with a single frame canvas (click to watch, click to hide)
// @include       http://chat.stackexchange.com/rooms/*
// @include       http://chat.stackoverflow.com/rooms/*
// ==/UserScript==


function replaceGif() {
	var gif = this;
    var canvas = document.createElement('canvas'),
        context = canvas.getContext('2d'),
		width = canvas.width = (gif.width > 300 ? 300 : gif.width ),
        height = canvas.height = (gif.height > 166 ? 166 : gif.height );
	
    context.drawImage(gif, 0, 0);
    context.font = "20px arial";
    context.fillStyle = "white";
    context.fillText("Click to play", width / 2 - 52, height / 2 + 8);

    function tGif() {
        gif.parentNode.insertBefore(canvas, gif);
        gif.parentNode.removeChild(gif);
        return false;
    }

    function tCanvas() {
        canvas.parentNode.insertBefore(gif, canvas);
        canvas.parentNode.removeChild(canvas);
        return false;
    }

    canvas.onclick = tCanvas;
    gif.onclick = tGif;

    gif.setAttribute('data-replacegif', true);

    tGif();
}

function scanChat() {
	var images = chat.getElementsByTagName('img');
	[].forEach.call(images, function(img) {
		
		var sp = img.src.split('?')[0],
			ext = sp.substr(-3);
		if( (ext.toLowerCase() === 'gif') && (img.getAttribute('data-replacegif') !== true) ) {
			img.onload = replaceGif.bind(img);
		}
		
	});
}

var chat = document.getElementById('chat');
chat.addEventListener('DOMSubtreeModified', scanChat, false);
scanChat();