Edit in JSFiddle

/*
 * jQuery firefly plugin 0.2
 *
 *
 * Copyright (c) 2010 Dharmveer Motyar
 * http://motyar.blogspot.com
 * 
 * $Id$
 * 
 * licensed under the MIT licenses:
 *   http://www.opensource.org/licenses/mit-license.php
 *  
 *
 * Creates a firefly effect.
 *
 * @example $.firefly();
 *
 * @name firefly
 * @type jQuery
 * @cat Plugins/firefly
 */
(function($) {
/*
     * Plugin defaults 
     */
    var defaults = {
        images: ['http://dharmmotyar.googlecode.com/svn/trunk/images/spark.png', 'http://dharmmotyar.googlecode.com/svn/trunk/images//spark2.png',
                        'http://dharmmotyar.googlecode.com/svn/trunk/images/spark3.png', 'http://dharmmotyar.googlecode.com/svn/trunk/images/spark4.png'],
        total: 10,
        ofTop: 0,
        ofLeft: 0,
        on: 'document.body'

    };


    $.firefly = function(settings) {
        $.firefly.settings = $.extend({}, defaults, settings);
        $.firefly.eleHeight = $($.firefly.settings.on).height();
        $.firefly.eleWidth = $($.firefly.settings.on).width();
        if ($.firefly.settings.on != 'document.body') {
            var off = $($.firefly.settings.on).offset();
            $.firefly.offsetTop = off.top;
            $.firefly.offsetLeft = off.left;
            $.firefly.eleHeight = $($.firefly.settings.on).height();
            $.firefly.eleWidth = $($.firefly.settings.on).width();
        }
        else {
            $.firefly.offsetTop = 0;
            $.firefly.offsetLeft = 0;
            $.firefly.eleHeight = $(document.body).height();
            $.firefly.eleWidth = $(document.body).width();

        }



        if ($.firefly.preloadImages()) {
            for (i = 0; i < $.firefly.settings.total; i++) {
                $.firefly.fly($.firefly.create($.firefly.settings.images[$.firefly.random(($.firefly.settings.images).length)]));
            }
        }
        return;
    };

/*
     * Public Functions
     */

    $.firefly.create = function(img) {
        spark = $('<img>').attr({
            'src': img
        }).hide();

        $("#header").append(spark);
        return spark.css({
            'position': 'absolute',

            'z-index': -1 * $.firefly.random(20),
            //under all the stuff
            top: $.firefly.offsetTop + $.firefly.random(($.firefly.eleHeight - 50)),
            //offsets
            left: $.firefly.offsetLeft + $.firefly.random(($.firefly.eleWidth - 50)) //offsets
        }).show();
    }



    $.firefly.fly = function(sp) {

        $(sp).animate({
            top: $.firefly.offsetTop + $.firefly.random(($.firefly.eleHeight - 50)),
            //offsets
            left: $.firefly.offsetLeft + $.firefly.random(($.firefly.eleWidth - 50))
        }, (($.firefly.random(10) + 5) * 2000), function() {
            $.firefly.fly(sp)
        });
    };

    $.firefly.preloadImages = function() {
        var preloads = new Object();
        for (i = 0; i < ($.firefly.settings.images).length; i++) {
            preloads[i] = new Image();
            preloads[i].src = $.firefly.settings.images[i];
        }
        return true;
    }

    $.firefly.random = function(max) {
        return Math.ceil(Math.random() * max) - 1;
    }

})(jQuery);


$.firefly({
    images: ['http://www.itsfirefly.com/images/fly1by1.png', 'http://www.itsfirefly.com/images/fly2by2.png'],
    //Fly images        
    total: 165,
    //number of flies
    on: '#header' //id of div
});

body{
background:#111;
}
#header{
width:600px;
height:400px;
margin:25px;
}
p{
color:#000;
text-shadow:0px 0px 35px #fff;
font-family: 'Corben', cursive;
font-size:80px;
opacity:0.06;
margin:50px;
text-align:center;
}

<link href='http://fonts.googleapis.com/css?family=Corben:700' rel='stylesheet' type='text/css'>
<div id="header"><p>Firefly<br />kachibito</p</div>