window.onresize = resize;

function resize() {
    var myWidth = 0;
    var myHeight = 0;
    //var Width = 2048, Height = 1536;
    var Width = 1500, Height = 998;
    var AspectRatioImg = Width / Height;

    var img = document.getElementById("imgBig");


    if (typeof (window.innerWidth) == 'number') {
        //Non-IE
        myWidth = window.innerWidth;
        myHeight = window.innerHeight;
    }
    else if (document.documentElement && (document.documentElement.clientWidth || document.documentElement.clientHeight)) {
    //IE 6+ in 'standards compliant mode'
        myWidth = document.documentElement.clientWidth;
        myHeight = document.documentElement.clientHeight;
    }
    else if (document.body && (document.body.clientWidth || document.body.clientHeight)) {
    //IE  compatible
        myWidth = document.body.clientWidth;
        myHeight = document.body.clientHeight;
    }

    var AspectRatioWindow = myWidth / myHeight;

    if (AspectRatioImg < AspectRatioWindow) {
        img.width = myWidth;
        img.height = myWidth / AspectRatioImg;
    }
    else {
        img.width = myHeight * AspectRatioImg;
        img.height = myHeight;
    }
        
//    img.width = myWidth;
//       img.height = myHeight;
}