bgResize.js

/*
* Plugin Name : imgResize
* Author      : Shane Donnelly
*/
(function($){

  //here's our resizer object
  $.Resize = function (el, options) {
      this.options = options;
      this.$el = $(el);
      this.init();
  };
  
  //initialize the plugin
  $.Resize.prototype.init = function () {
    var base = this;
    base.settings = $.extend({}, base.defaults, base.options);
    
    base.image = $('<img />')
    .attr('src', base.settings.img_url)
    .attr('style', 'position:fixed;display:block;top:'+ base.settings.top_offset +'px;')
    .load(function(){
      base.$el
      .css('background','none')
      .append(base.image);
      base.img_width = base.image.width();
      base.img_height = base.image.height();
      base.scaleImage();
      
      $(window).resize(function(){
        base.scaleImage();
      });
    });
  }
  
 $.Resize.prototype.scaleImage = function(){
    var base = this, 
        window_width = $(window).width(),
        window_height = $(window).height(),
        width_ratio = window_width/base.img_width,
        height_ratio = window_height/base.img_height,
        temp_height = 0,
        temp_width = 0,
        left = 0;

    if(width_ratio > height_ratio){
    //make image width equal windowWidth
      base.$el.find('img').css('width',window_width);
      temp_height = base.img_height * width_ratio;
      base.$el.find('img').css('height',temp_height);
      base.$el.find('img').css('left', left);
    }
    else{
      base.$el.find('img').css('height',window_height);
      temp_width = base.img_width * height_ratio;
      base.$el.find('img').css('width',temp_width);

      if(base.settings.position === 'center'){
        left = (temp_width - window_width)/2;
        base.$el.find('img').css('left','-'+ left + 'px');
      }
      else{ //position === 'left'
        base.$el.find('img').css('left', left);
      }
    }
  }

  //default plugin options
  $.Resize.prototype.defaults = {
    img_url       : '',
    top_offset    : 0,
    position      : 'center' //left, center
  };
  
  //the jQuery plugin call
  $.fn.imgResize = function(options) {
    return this.each(function() {
        new $.Resize(this, options);
    });
  };
  
})(jQuery);