In the code snippet below, from my jQuery setup, I need to check if the image file actually exists and if not, I'd like to substitute a default image. Currently if the file does not exist, I just get a broken image placeholder...

$('#myTheme').change
(
    function() 
    {
    var myImage = $('#myTheme :selected').text();
    $('.selectedImage img').attr('src','../wp-content/themes/myTheme/styles/'+myImage+'/screenshot.jpg');
    //if screenshot.jpg does not exist, use "../../default.jpg" instead
    }
);