<img src="#" data-tablet="img/xl.jpg" data-laptop="img/xllaptop.jpg" >
<img  src="#" data-tablet="img/x2.jpg" data-laptop="img/x2laptop.jpg" >

var  currentWindowSize = $window.width();
    if (currentWindowSize <640)
{
        $(img).attr("src","img/low.jpg");
    }
    if (currentWindowSize >640)
{
        $(img).attr("src","img/high.jpg");
    }