JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<link rel="stylesheet" href="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">

JavaScript

/*Minified bookmarklet - copy & paste to Chrome Bookmark:

javascript:function defer(e){window.jQuery?e():setTimeout(function(){defer(e)},50)}setTimeout(function(){var e=document.createElement("script");e.src="//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js",document.getElementsByTagName("head")[0].appendChild(e)},1);var inputSelector=prompt("Enter selector for image you want to know the timestamp");defer(function(){var e=jQuery(inputSelector)[0];jQuery.get(e.src,function(e,t,n){var o=n.getResponseHeader("Last-Modified");alert("element was last modified: "+o)})});

*/

/*
raw code below
*/
//this will defer our calls until jQuery has been loaded
function defer(method) {
    if (window.jQuery) {
        method();
    } else {
        setTimeout(function() { defer(method) }, 50);
    }
}
//load jQuery asyncronous
setTimeout(function(){
var j = document.createElement('script'); j.src = "//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"; 
document.getElementsByTagName('head')[0].appendChild(j);
},1);

//input prompt for the jquery selection (e.g. img#myname123 or img.thumbnail)
//it will use the first result found, if nothing is found the script will simply do nothing
var inputSelector = prompt("Enter selector for image you want to know the timestamp");

//defer main-function
defer(function () {
var img = jQuery(inputSelector)[0];
jQuery.get(img.src,function(data,status,xhr){
    // in the callback - after the ajax request completes...
    var filetime = xhr.getResponseHeader('Last-Modified');
    //output last-modified header-field via alert
    alert("element was last modified: " + filetime);
});
});