DOMAssistant.DOMReady( function() {
    $("a[href$=.png]").addEvent("click", function() {
        var newImage = $$('canvas').create('img', {src: this.href}, true).setStyle('opacity', 0);
        $('#canvas img').first().animate({opacity: 0}, {duration:500, callback: function() { this.remove();}});
        newImage.animate({opacity: 1}, {duration:500});
        return false;
    } );
} );