学习自http://www.daqianduan.com/6419.html
单张图片(图片在文档中)

// HTML
<img id='xiu' src=""> 
//js
 $(document).ready(function(){

    //jquery
    $('#xiu').load(function(){
       // 加载完成 
    });

   //原生  onload
    var xiu = document.getElementById('xiu')
    xiu.onload = xiu.onreadystatechange = function(){
       if(!this.readyState||this.readyState=='loaded'||this.readyState=='complete'){
           // 加载完成 
       }
    };

})

注：
1、IE8及以下版本不支持onload事件，但支持onreadystatechange事件；
2、readyState是onreadystatechange事件的一个状态，值为loaded或complete的时候，表示已经加载完毕。
3、以下内容省略兼容

单张图片(图片动态创建)

//js
 var xiu = new Image()
 xiu.src = 'http://www.daqianduan.com/wp-content/uploads/2014/11/hs-xiu.jpg'
 xiu.onload = function(){
    // 加载完成 
 }

单张图片(结合Promise)

//js
 new Promise((resolve, reject)=>{
    let xiu = new Image()
    xiu.src = 'http://www.daqianduan.com/wp-content/uploads/2014/11/hs-xiu.jpg'
    xiu.onload = function(){
       // 加载完成 
       resolve(xiu)
    }
 }).then((xiu)=>{
 //code
 })

多张图片

var img = [],  
    flag = 0, 
    mulitImg = [
    'http://www.daqianduan.com/wp-content/uploads/2017/03/IMG_0119.jpg',
    'http://www.daqianduan.com/wp-content/uploads/2017/01/1.jpg',
    'http://www.daqianduan.com/wp-content/uploads/2015/11/jquery.jpg',
    'http://www.daqianduan.com/wp-content/uploads/2015/10/maid.jpg'
 ];
 var imgTotal = mulitImg.length;
 for(var i = 0 ; i < imgTotal ; i++){
    img[i] = new Image()
    img[i].src = mulitImg[i]
    img[i].onload = function(){
       //第i张图片加载完成
       flag++
       if( flag == imgTotal ){
          //全部加载完成
       }
    }
 }

多张图片(结合Promise)

  let mulitImg = [
     'http://www.daqianduan.com/wp-content/uploads/2017/03/IMG_0119.jpg',
     'http://www.daqianduan.com/wp-content/uploads/2017/01/1.jpg',
     'http://www.daqianduan.com/wp-content/uploads/2015/11/jquery.jpg',
     'http://www.daqianduan.com/wp-content/uploads/2015/10/maid.jpg'
 ];
 let promiseAll = [], img = [], imgTotal = mulitImg.length;
 for(let i = 0 ; i < imgTotal ; i++){
     promiseAll[i] = new Promise((resolve, reject)=>{
         img[i] = new Image()
         img[i].src = mulitImg[i]
         img[i].onload = function(){
              //第i张加载完成
              resolve(img[i])
         }
     })
 }
 Promise.all(promiseAll).then((img)=>{
     //全部加载完成
 })