关于一段Jquery代码，不报错但是莫名失效问题

//这里动态添加了图片

function imginit(){

  var top = 150;

  var left = new Array(650,950);

  for(var i = 0; i < 10; i++){

    var images = document.getElementById('images');

    var tempimg = document.createElement('img');

    tempimg.setAttribute('class','img'+(i+1));

    tempimg.setAttribute('src','css/images/img'+(i+1)+'.jpg');

    tempimg.setAttribute('style','top:'+(top+(Math.floor(i/2)*280))+'px;left:'+left[i%2]+'px;');

    images.append(tempimg);

  }

}

window.onload = function(){

  imginit();}

//选择图片功能，实现将图片拖动到一个目标区域//突然失效//这段代码写在$(document).ready(function{})里了

    $('img[class^=img]').on({

      mousedown:function(e){

        var img = $(this);

        var os = img.offset();

        sessionStorage.setItem("left",os.left);

        sessionStorage.setItem("top",os.top);

        e.preventDefault();

        dx = e.pageX - os.left;

        dy = e.pageY - os.top;

        $(document).on('mousemove.drag',function(e){

          sessionStorage.setItem("cleft",e.pageX - dx);

          sessionStorage.setItem("ctop",e.pageY - dy)

          img.offset({top:e.pageY - dy,left:e.pageX - dx});

          $("#target").text("请放这里！");//目标区域是一个div//高度宽度都是144px

        });

      },

      mouseup:function(e){

        var target = $('#target');

        var tos = target.offset();//获取目标区域的top和left

        if((tos.left+250) >= sessionStorage.getItem("cleft") && (tos.left-250) <=sessionStorage.getItem("cleft")

      && (tos.top+250) >= sessionStorage.getItem("ctop") && (tos.top-250) <= sessionStorage.getItem("ctop")){

//判断图片是否接触到目标区域

        target.css('background',"url("+$(this).prop('src')+")");

        target.css('background-size','144px 144px');

      }

      target.text("");

      sessionStorage.setItem('picture',"images/"+$(this).prop('class')+".jpg");

        e.preventDefault();

        $(document).off('mousemove.drag');

        $(this).offset({top:sessionStorage.getItem("top"),left:sessionStorage.getItem("left")});//让拖动的图片弹回原位置

      }

    });