jQuery的.unload（）不起作用

Question

Any idea why this code works:

$.ajax({
    type: 'GET', 
    url: 'delete_image.php'
});

And this one doesn't:

$(window).unload(function() {
    $.ajax({
        type: 'GET', 
        url: 'delete_image.php' 
    });
});

In delete_image.php, I just have:

unlink('test.jpg');