via http://userscripts.org/scripts/source/43621.user.js?

tumblr_dashboard_high-re.user.js

// ==UserScript==
// @name           Tumblr Dashboard High-Res Photos
// @namespace      http://cxx.tumblr.com/
// @include        http://www.tumblr.com/*
// @version        0.0.5.2.20101020
// ==/UserScript==
// original : http://userscripts.org/scripts/source/43621.user.js?

(function(){
    function $(selectors, context) {
	return Array.prototype.slice.call((context || document).querySelectorAll(selectors), 0);
    }

    function replaceImage(post) {
	var img = $('img[width="500"], img[height="700"]', post)[0];
	if (!img || !img.src.match(/(\w+)o1_500\./))
	    return;
	var hash = RegExp.$1;
	var postId = post.id.replace('post_', '');
	var host = $('a.post_avatar', post)[0].href;
	var highResPath = host + 'photo/1280/' + postId + '/1/' + hash;
	img.src = highResPath;
	img.style.width = img.style.height = 'auto';
	img.style.maxWidth = '999999px';
	
    }

    $('#posts')[0].addEventListener('DOMNodeInserted', function(e) {
	replaceImage(e.target);
    }, false);

    var container = $('#container')[0];
    if (container)
	container.style.marginLeft = '0';
    $('li.post').forEach(replaceImage);
})();