function loadImages(sources, callback){
    var images = {};
    var loadedImages = 0;
    var numImages = 0;
    // get num of sources
    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        images[src] = new Image();
        images[src].onload = function(){
            if (++loadedImages >= numImages) {
                callback(images);
            }
        };
        images[src].src = sources[src];
    }
}
 
window.onload = function(images){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
 
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };
 
    loadImages(sources, function(images){
        context.drawImage(images.darthVader, 100, 30, 200, 137);
        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};