Copy this link and create bookmark with this as it's URL in your bookmark toolbar:
javascript:(function(){"use strict";function t(t){function l(e){var t=new XMLHttpRequest;t.onload=function(){if(this.status>=200&&this.status<400){a.push(e)}};t.onloadend=c;t.open("HEAD",r+e+s,true);t.send();++f}function c(){--f;if(f===0){if(a.length>0){a.sort(function(e,t){return e-t});var e=r+a[a.length-1]+s;location.href=e}else{alert("No bigger image found.")}}}var n=/^(.*_)(\d+)(\.[a-z]+)$/i.exec(t);if(!n){alert("Not a tumblr image URL.");return}var r=n[1];var i=Number(n[2]);var s=n[3];var o=null;for(var u=0;u<e.length;++u){if(e[u]>i){o=u;break}}if(o===null){alert("There is no bigger known size.");return}var a=[];var f=0;for(var u=o;u<e.length;++u){l(e[u])}}var e=[75,100,250,400,500,1280];t(location.href)})()
Then when you view a tumblr image and suspect that there is a bigger version of it click this bookmarklet.