Need help on a simple Jquery practice

I've just started to practice Jquery at these days. Could you please have a take look at the below link and give me some advise on how to validate my input for only jpeg extension.

$('#submit').click( function() {
var val = $('#URL').val();


if(val !== "") {
    $('#URL').val(function (){
    $('#frame').fadeOut(800, function(){
        $(this).css({backgroundImage: "url("+val+")"}); 
        $(this).fadeIn(800);
    });

});
}
else{
    $('#lightbox, #feedback').slideDown(700, function(){
        $("body").click(function() {
        $('#lightbox, #feedback').slideUp(700);
        return false;
        }); 
    });
}
return false;

Thanks in advance.

Regards.

Özgün