direct.js

$('#myImage').resizeToParent();

resizeimagetoparent.js

/*
 * Plugin Name: Resize Image to Parent Container
 *
 * Author: Christian Varga
 * Author URI: http://christianvarga.com
 * Plugin Source: https://github.com/levymetal/jquery-resize-image-to-parent/
 *
 */

(function($) {
  $.fn.resizeToParent = function(opts) {
    var defaults = {
     parent: 'div',
     delay: 100
    }

    var opts = $.extend(defaults, opts);

    function positionImage(obj) {
      // reset image (in case we're calling this a second time, for example on resize)
      obj.css({'width': '', 'height': '', 'margin-left': '', 'margin-top': ''});

      // dimensions of the parent
      var parentWidth = obj.parents(opts.parent).width();
      var parentHeight = obj.parents(opts.parent).height();

      // dimensions of the image
      var imageWidth = obj.width();
      var imageHeight = obj.height();

      // step 1 - calculate the percentage difference between image width and container width
      var diff = imageWidth / parentWidth;

      // step 2 - if height divided by difference is smaller than container height, resize by height. otherwise resize by width
      if ((imageHeight / diff) < parentHeight) {
       obj.css({'width': 'auto', 'height': parentHeight});

       // set image variables to new dimensions
       imageWidth = imageWidth / (imageHeight / parentHeight);
       imageHeight = parentHeight;
      }
      else {
       obj.css({'height': 'auto', 'width': parentWidth});

       // set image variables to new dimensions
       imageWidth = parentWidth;
       imageHeight = imageHeight / diff;
      }

      // step 3 - center image in container
      var leftOffset = (imageWidth - parentWidth) / -2;
      var topOffset = (imageHeight - parentHeight) / -2;

      obj.css({'margin-left': leftOffset, 'margin-top': topOffset});
    }

    // run the position function on window resize (to make it responsive)
    var tid;
    var elems = this;

    $(window).on('resize', function() {
      clearTimeout(tid);
      tid = setTimeout(function() {
        elems.each(function() {
          positionImage($(this));
        });
      }, opts.delay);
    });

    return this.each(function() {
      var obj = $(this);

      // hack to force ie to run the load function... ridiculous bug 
      // http://stackoverflow.com/questions/7137737/ie9-problems-with-jquery-load-event-not-firing
      obj.attr("src", obj.attr("src"));

      // bind to load of image
      obj.load(function() {
        positionImage(obj);
      });

      // run the position function if the image is cached
      if (this.complete) {
        positionImage(obj);
      }
    });
  }
})( jQuery );

with-parent.js

$('#myImage').resizeToParent({parent: '.parentContainer'});