http://d.hatena.ne.jp/Griever/20090428/1240923824

Imitation lightbox.user.js

// ==UserScript==
// @name           Imitation lightbox
// @namespace      http://d.hatena.ne.jp/Griever/
// @include        http://*
// @run-at         document-start
// ==/UserScript==

({
	STOP_TARGET_FRAME: true, // 他のフレームで開くリンクは無視
	REMOVED_SCROLL: true, // 画像をクリックして消したあと元の位置にスクロールする
	IMAGE_RESIZING: true, // 大きい画像を画面内に縮小する

	REGEXP: [
		{
			name: 'Google Images'
			,url: /^.*?imgurl=(https?:\/\/.+?)\&.*?$/i
			,newURL: "$1"
		}
		,{
			name: 'akiba-pc.watch.impress.co.jp'
			,url: /^(http:\/\/akiba\-pc\.watch\.impress\.co\.jp\/hotline\/.*)\.html$/i
			,newURL: "$1.jpg"
		}
		,{
			name: 'impress'
			,url: /^(http:\/\/[^/]+\.impress\.co\.jp\/img\/.*?\/)html\/(\w+\.jpg)\.html.*?$/i
			,newURL: "$1$2"
		}
		,{
			name: 'ITMedia'
			,url: /^(http:\/\/image\.itmedia\.co\.jp\/)\w\/\w+\/(.*\.jpg).*?$/i
			,newURL: "$1$2"
		}
		,{
			name: 'journal.mycom'
			,url: /^http:\/\/journal\.mycom\.co\.jp\/photo\/(.*\.jpg).*?$/i
			,newURL: "http://j.mycom.jp/$1"
		}
		,{
			name: 'http://mantan-web.jp'
			,url: /^(http:\/\/mantan-web\.jp\/)(gallery\/.*?\/)([1-9]+).html.*?$/i
			,newURL: "$1assets/images/$20$3.jpg"
		}
		,{
			name: 'asablo.jp'
			,url: /^(http:\/\/.+asablo.jp\/blog\/)imgview(\/.+\.(?:jpe?g|gif|png|bmp))\.html.*?$/i
			,newURL: "$1img$2"
		}
		,{
			name: 'Yahoo Blogs'
			,url: /^.*?(http:\/\/img[0-9]?\.blogs\.yahoo\.co\.jp\/.*?\/img_[0-9_]+).*?$/i
			,newURL: "$1"
		}
		,{
			name: '2ch etc'
			,url: /^(http:\/\/)(?:ime\.nu|pinktower\.com)\/(.*\.(?:jpe?g|gif|png|bmp)).*?$/i
			,newURL: "$1$2"
		}
		,{
			url: /^.*?(https?:\/\/(?:[^\/][^\/]|[^#]+)\.(?:jpe?g|gif|png|bmp)).*?$/i
			,newURL: "$1"
		}
	],

	get imageTemplate() {
		var img = document.createElement('img');
		img.alt = 'img';
		img.className = 'imitation-lightbox-image';
		img.style.cssText = 'position:absolute; z-index:20000; border:1px solid blue;';
		if (this.IMAGE_RESIZING) {
			img.style.maxWidth = '100%';
			img.style.maxHeight = '100%';
			img.style.WebkitBoxSizing = 
			img.style.OBoxSizing = 
			img.style.MozBoxSizing = 
			img.style.boxSizing = 'border-box';
		}
		delete this.imageTemplate;
		return this.imageTemplate = img;
	},

	isChrome: !!window.chrome,
	init: function() {
		this.REGEXP.forEach(function(e){
			if (typeof e.url == 'string')
				e.url = new RegExp(e.url, 'i');
		});
		
		document.addEventListener('click', this, true);
		document.addEventListener((this.isChrome? 'keydown' : 'keypress'), this, true);
	},

	handleEvent: function(event) {
		switch(event.type) {
			case 'click'   : this.onClick(event); break;
			case 'keydown' : this.onKeypress(event); break;
			case 'keypress': this.onKeypress(event); break;
			case 'error'   : this.onError(event); break;
		}
	},

	onClick: function(event) {
		if (event.button != 0) return;
		if (event.ctrlKey || event.shiftKey || event.altKey) return;
		var elem = event.target;
		if (elem.className.indexOf('imitation-lightbox-image') >= 0) {
			if (this.REMOVED_SCROLL)
				elem.scrollIntoView(true);
			elem.parentNode.removeChild(elem);
			event.preventDefault();
			event.stopPropagation();
			return;
		}

		do {
			if (typeof elem.hasAttribute === 'function' && elem.hasAttribute('href'))
				break;
		} while(elem = elem.parentNode);
		if (!elem) return;

		if (!this.STOP_TARGET_FRAME) {
			var target = elem.getAttribute('target');
			if (target && target !== '_blank' && target !== 'self')
				return;
		}

		var href = elem.href;
		var info;
		this.REGEXP.some(function(e){
			if (e.url.test(href)){
				info = e;
				return true;
			}
		});
		if (!info) return;
		event.preventDefault();
		event.stopPropagation();
		var doc = elem.ownerDocument;
		var win = doc.defaultView;
		var img = this.imageTemplate.cloneNode(false);
		img.src = href.replace(info.url, info.newURL);
		img.style.top = win.pageYOffset + 'px';
		img.style.left = win.pageXOffset + 'px';
		img.addEventListener('error', this, false);
		doc.body.appendChild(img);
	},
	
	onKeypress: function(event) {
		if (event.keyCode == 27 && !event.shiftKey && !event.ctrlKey && !event.altKey) {
			var imgs = Array.prototype.slice.call(document.getElementsByClassName('imitation-lightbox-image'));
			imgs.forEach(function(e) {
				e.parentNode.removeChild(e);
			});
		}
	},

	onError: function(event) {
		var elem = event.target;
		elem.parentNode.removeChild(elem);
		window.status = 'Error:' + elem.href;
	}

}).init();

Firefox, Opera, Chrome で動くように書いたつもり。
Chrome か Opera で拡張子.js で実行するとページ読み込み前から動作する