var imageWidth = 1600;
  var imageHeight = 1200;
  var aspectRatio = 1.333;

window.onresize = resizeHandler;


function resizeHandler() {
  
    var browserWidth;
    var browserHeight;
    
    if (typeof(window.innerWidth) == 'number') {
        browserWidth = window.innerWidth;
        browserHeight = window.innerHeight;
    } else if (document.documentElement && (document.documentElement.clientWidth || document.documentElement.clientHeight)) {
        browserWidth = document.documentElement.clientWidth;
        browserHeight = document.documentElement.clientHeight;
    } else if (document.body && (document.body.clientWidth || document.body.clientHeight)) {
        browserWidth = document.body.clientWidth;
        browserHeight = document.body.clientHeight;
    }

  var ratio = browserWidth / browserHeight;
  var newWidth;
  var newHeight;
  
  if (ratio >= aspectRatio) {
    newWidth = browserWidth;
    newHeight = imageHeight / imageWidth * browserWidth;
  } else {
    newWidth = imageWidth / imageHeight * browserHeight;
    newHeight = browserHeight;
  }
  
  
  var image = document.getElementById('currentPhoto');
    image.style.width = newWidth + 'px';
    image.style.height = newHeight + 'px';
}