originalMain.js

var comment = $('.imageComment').val();
    //takes the value of the comment field and clears it
    $('.imageComment').val('');
    var url = $('.imageURL').val();
    $('.imageURL').val('');

    //creates a new instance of the constructor POST passing the three assigned variables
    var outgoingPost = new Post();

    outgoingPost.set({
        'URL'     :   $('.imageURL').val(),
        'Comment' :   $('.imageComment').val()

    });

you're capturing a value and assigning it to url and comment respectively. however, within the 'set' you're assigning them to the values....that you just cleared out. so instead it should be

 outgoingPost.set({
    'URL'     :   url,
    'Comment' :   comment

});