backbone.js , waiting for external post request to complete - ajax

Question

I have an image uploader on my index.php page, when the image starts uploading (initiated by dragging image onto upload box) , the POST request is created (FYI HTML5 + php Uploader)
POST http://localhost:8888/backbone_images/post_file.php
Inside of my post_file.php , I retrieve the URL of my uploaded image by inserting it in my exit_status, than later in my script assigning it to a global javascript variable 'globe'
if(move_uploaded_file($pic['tmp_name'], $upload_dir.$pic['name'])){
exit_status($pic['name']);
}
Inside of my backbone script file, I check for the drop event into the dropbox, which works,
App.Views.AddImage = Backbone.View.extend({
el: '#addImage',
events: {
'drop': 'submit'
},
so the image has been dragged onto the dropbox, and I call my submit function to create a new model
and assign the url of the new image (which I pass through a javascript variable)
to the url of my new model
submit: function(e) {
e.preventDefault(); // preventing default submission..
var newImage = globe; // assign globe --url of the newly created image--to new model
var imager = new App.Models.Person({ url: newImage });//creating a new img object
this.collection.add(imager);
}
The problem is that when my backbone submit function is triggered after an image is dropped, my POST to /post_file.php hasn't run yet, as a result my variable 'globe' doesn't contain the new URL, and my newly created model ends up lacking a url
So question: is there any way for me to wait for the POST to finish within my backbone events before executing my 'submit' function, after an image drop?
I tried to use: $.when( $.ajax("/post_file.php") ) inside of my submit function, but all this does is run another POST request, leaves my variable value, than run the original POST request retrieving the image URL that I'm waiting for.
My preferred workflow would be
a. drag/upload image, (POST performs)
b. new model is added to
collection with URL of uploaded image,
but I end having to add an additional click function first otherwise the POST doesn't finish
a. drag/upload image, (POST peforms)
b. click on a submit button
c. model is added to collection