I perfectly know that the best way to protect image download is not putting it on internet in the first place. But sometimes there are bad days...

image_protector.js

//Development version:
function prtctImg() {
    var protected_images = document.getElementsByTagName("img");
    for (index = 0; index < protected_images.length; ++index) {
        protected_images[index].oncopy = function() {return false;};
        protected_images[index].oncut = function() {return false;};
        protected_images[index].onpaste = function() {return false;};
        protected_images[index].ondrag = function() {return false;};
        protected_images[index].ondragstart = function() {return false;};
        protected_images[index].oncontextmenu = function() {return false;};
    };
};
prtctImg();
var target = document.querySelector('body');
var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
        prtctImg();
    });    
});
var config = {subtree: true, childList: true }
observer.observe(target, config);

image_protector.min.js

//Minified version:
function prtctImg(){var n=document.getElementsByTagName("img");for(index=0;index<n.length;++index)n[index].oncopy=function(){return!1},n[index].oncut=function(){return!1},n[index].onpaste=function(){return!1},n[index].ondrag=function(){return!1},n[index].ondragstart=function(){return!1},n[index].oncontextmenu=function(){return!1}}prtctImg();var target=document.querySelector("body"),observer=new MutationObserver(function(n){n.forEach(function(n){prtctImg()})}),config={subtree:!0,childList:!0};observer.observe(target,config);