Ruby on Rails how to not lose user input after using oauth authentication

I am working on a Ruby on Rails app that someone else wrote, and the client now wants changed it with a specific flow of picture creation. The flow goes like this:

User goes to www.website.com/pictures/new and sees a form with only a URL field and a submit button, looking like this:

  <%= form_for @picture do |f| %>
    <%= tag :input, id: 'url-field', size: 30, title: "Submit the URL of your picture" %>
    <%= f.hidden_field :url %>
    <%= f.submit 'Submit', id: 'url-submit', class: 'blue-btn' %>
  <% end %>

When the user submits a url, the form is intercepted by Jquery instead of posting as it normally would. Currently it is posting to a validate action on the controller, like so:

MYAPP.picturesController.newAction = {
  init: function() {
  var initVars = {
    $newPictureForm: $('#new_picture')
    $urlSubmitBtn: $('#url-submit')
  };
  initVars.$newPictureForm.submit(function() {
  initVars.$urlSubmitBtn.attr('disabled', 'disabled');
  $.post(MYAPP.picturesController.newAction.validationPath, initVars.$newPictureForm.serialize(), function(response) {
    initVars.$urlSubmitBtn.removeAttr('disabled');
    if (response.status == 'ok') {
      initVars.$flashMsg.empty();
      $.fn.colorbox({
        inline: true,
        scrolling: false,
        href: '#viewer',
        onCleanup: function() {
          if ($.browser.msie) {
            $('#site-player').removeAttr('src');
          }
        },
        onOpen: function() {
          $('#site-player').attr('src', response.picture.embed_url);
        },
        onLoad: function() {
          $('#colorbox').removeClass('cboxDialog');
        }
      });
    } else {
      initVars.$flashMsg.text(response.error);
    }
  }, 'json');
  return false;
});

After it has verified the url, it comes back and displays two links: Log in with Twitter and Log in with Facebook. I am using Oauth to complete this. A user then clicks one of those and goes through the verification process. Here is my problem: when the user clicks one of those links, I lose the URL the user submitted. I would usually change up the flow and make the user log in first, but this is exactly how the client wants it.

I know with Oauth you can pass variables to Facebook/Twitter and use them upon returning to your app, like so

<%= link_to "Sign in with Facebook", user_omniauth_authorize_path(:facebook, url: @picture.url)  %> <br />
<%= link_to "Sign in with Twitter", user_omniauth_authorize_path(:twitter, url: @picture.url) %>

but at this point, @picture doesn't contain the URL because it hasn't been posted. And as far as I can tell the POST request jquery makes doesn't set it, here is the validate action:

def validate
  @picture = Picture.new(params[:picture])
  respond_to do |format|
    format.json do
      if @picture.valid? # for this post just assume this will return true
        render json: { status: :ok, picture: { embed_url: @picture.embed_url }
      else
        render json: { status: :error, error: @picture.all_errors.first }
      end
    end
  end
end

So my problem is that I need to submit the URL to facebook or twitter so I can create the picture once they've come back, but I can't figure out how to get the URL since it is never posted except as an AJAX post.

I've thought about setting @picture to a global variable $picture in the validate action so I can access $picture in the view, but that seems like it isn't the right solution and could be buggy.

So does anyone have any advice on how I can extract the URL to send with the facebook or twitter request?

Let me know if you need to see any more code.. thanks a ton in advance.