Force tumblr posts use hight quality images. Tempermonkey user script.

user.js

// ==UserScript==
// @name         Tumblr image size converter
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Force tumblr post photos use high quality
// @author       Alashov Berkeli
// @include      https://www.tumblr.com/*
// @require      https://ajax.googleapis.com/ajax/libs/jquery/1.7/jquery.min.js
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    String.prototype.contains = function(it) { return this.indexOf(it) != -1; };
    String.prototype.replaceAll = function(search, replacement) {
        var target = this;
        return target.replace(new RegExp(search, 'g'), replacement);
    };
    function isString(str){
        return typeof str === 'string' || str instanceof String;
    }

    var sizes = ['_540.', '_500.', '_400.', '_250.', '_100.' ];

    function convertSizes(sel){
        var j = jQuery;
        var images = j(sel);

        j.each(images, function(e){
            var e = j(images[e]);

            var oldSrc = e.attr("src");
            if (oldSrc !== undefined && isString(oldSrc)) {
                var isGif = oldSrc.endsWith(".gif");

                if (! isGif) {
                    for (var i = sizes.length - 1; i >= 0; i--) {
                        if (oldSrc.contains(sizes[i])){
                            var newSrc = oldSrc.replaceAll(sizes[i], "_1280.");

                            e.attr("src", newSrc);
                            console.log("Converted to big size: " + newSrc);
                        } else {
                            console.log("Didn't convert because it has no size or already converted: " + oldSrc);
                        }
                    }
                }
            }
        });
    }

    convertSizes(".post_container img");

    jQuery("img").live("mouseover", function(e){
		convertSizes(".post_container img");
	});
})();