Edit in JSFiddle

function onloadHandler(){
　　alert('img loaded, img.src = ' + this.src);
};
var img = document.createElement('img');
img.src = 'http://im-img.qq.com/inc/images/new_header2/logo.gif';
if(img.complete){
    console.log('a');
    onloadHandler.call(img);
}else{
    var t1 = new Date() - 0;
    var div = document.createElement('div');
    for(var i=0; i<10000; i++){
        div.innerHTML = div.innerHTML + i;
    }
    document.body.appendChild(div);
    var t2 = new Date() - 0;
    console.log(t2 - t1);  //在chrome23.0里，6000++ms
    img.onload = onloadHandler;
    
    img.onload = onloadHandler;  //神奇的事情：onload事件触发，而且处理方法被执行了
}