http://gyazo.com/32b2816b1c2b2c26d9882b895b5eb436

fb-thumbnail-selector.user.js

// ==UserScript==
// @name        fb-thumbnail-selector
// @namespace   http://d.hatena.ne.jp/Cherenkov/
// @include     http://www.facebook.com/*
// @include     https://www.facebook.com/*
// @grant       GM_addStyle
// @grant       GM_log
// @version     0.1
// @date        20121010
// ==/UserScript==

function log(m) {
    if (unsafeWindow.console) {
        unsafeWindow.console.log.apply(this, arguments);
    } else {
        console.log.apply(this, arguments);
    }
}


// sample link:
// http://jigokuno.com/
// http://feeds.feedburner.com/Octocats


/*
//debug: auto input url
try {
	setTimeout(function(){
		var textarea = document.querySelector('.mentionsTextarea');
		if (textarea) {
			//var event = document.createEvent('MouseEvents');
			//event.initEvent('click', true, true);
			//textarea.dispatchEvent(event);

			//textarea.value = 'http://jigokuno.com/';
			textarea.value = 'http://feeds.feedburner.com/Octocats';
		}
	}, 2000);
} catch (e) {}
*/

function changeThumbnail(newUrl) {
	document.querySelector('form:not(.hidden_elem) .UIThumbPager_Thumbs img:not(.UIThumbPager_Hidden)').classList.add('UIThumbPager_Hidden');
	document.querySelector('form:not(.hidden_elem) .UIThumbPager_Thumbs img[src="' + newUrl + '"]').classList.remove('UIThumbPager_Hidden');
	var param = document.querySelector('form:not(.hidden_elem) input[name="attachment[params][images][0]"][value*="safe_image.php"]');
	param.value = newUrl;
}

var btn = document.createElement('div');
btn.id = 'GMfts_btn';
btn.innerHTML = '⬚';
btn.className = 'GMfts_hide';
document.body.appendChild(btn);
btn.onclick = function() {
	//viewerを複数生成しないようにする。
	var viewer = document.getElementById('GMfts_viewer');
	if (viewer) {
		viewer.parentNode.removeChild(viewer);
		return;
	}

	var div = document.createElement('div');
	div.id = 'GMfts_viewer';

	var df = document.createDocumentFragment();

	Array.forEach(document.querySelectorAll('form:not(.hidden_elem) .UIThumbPager_Thumbs img'), function(e) {
 		var img = e.cloneNode(true);
 		img.className = '';
 		img.setAttribute('style', '');
 		img.onclick = function() {
 			var newUrl = this.src;
 			changeThumbnail(newUrl);

 			setTimeout(function(){
 				div.classList.add('GMfts_hide');
				
				var viewer = document.getElementById('GMfts_viewer');
				viewer.parentNode.removeChild(viewer);
 			}, 200);
 		}
 		df.appendChild(img);
	});
	div.appendChild(df)
	document.body.appendChild(div);
}



//FB側で一度表示したページをキャッシュされると、MutationObserverで観測できない？




//var MutationObserver = unsafeWindow.MutationObserver || unsafeWindow.WebKitMutationObserver || unsafeWindow.MozMutationObserver;
  

// https://developer.mozilla.org/en-US/docs/DOM/MutationObserver
var observer = new MutationObserver(function(mutations) {
	mutations.forEach(function(mutation) {
		//console.log(mutation)
		for (var i = 0, n = mutation.addedNodes.length; i < n; i++) {
			var e = mutation.addedNodes[i];
			if (e.querySelector && e.querySelector('.UIShareStage_HasImage')) {
				var btn = document.getElementById('GMfts_btn');
				btn.classList.remove('GMfts_hide');
				e.querySelector('.UIThumbPagerControl_Text').appendChild(btn);
			}
			if (e.parentNode && e.parentNode.classList && e.parentNode.classList.contains('UIThumbPagerControl_PageNumber_Total')) {
				document.querySelector('#GMfts_btn').textContent = e.textContent;
			}
		}
	});
});
observer.observe(document.querySelector('#pagelet_composer, .timelineUnitContainer'), { childList: true, subtree: true, attributes: true });





GM_addStyle([
	'.GMfts_hide { display: none; }',
	'#GMfts_viewer { position: fixed; top: 50px; right: 10px; z-index: 999; background-color: azure; border: 4px solid sienna; width: 600px; height: 350px; overflow: scroll;}',
	'#GMfts_viewer img { max-width: 100px; vertical-align: middle; border: 2px solid white; margin: 1px;}',
	'#GMfts_viewer img:hover { border-color: red; }',

	'#GMfts_btn {',
		'color: white; font-size: 18px; width: 25px; padding: 0 4px; margin-left: 8px;',
		'text-align: center; display: inline; border-radius: 4px; vertical-align: middle;',
		'background: -moz-linear-gradient(top, #3e779d, #65a9d7); background: -webkit-linear-gradient(top, #3e779d, #65a9d7);',
	'}',
	'#GMfts_btn:hover { border-top-color: #28597a; background: #28597a; color: #ccc; }',
	'#GMfts_btn:active { border-top-color: #1b435e;  background: #1b435e; }',
].join(''));

firefoxにグリモンを入れていて暇な人向け。
上記のuser.jsファイルをfirefoxにドラッグアンドドロップでインストール。