Strange error uploading file

Hello, I'm just starting to learn ruby on rails and I have a problem
uploading pictures to my server.

I just want to store the url of the picture and upload its file to
public/images when I create a new Picture record. I don't know where is
my error and I am going crazy. I don't need it for any customer or
similar, I only want to understand why it doesn't work. Please help!

I have used this tutorial:
http://programming-ut.blogspot.com/2010/01/ruby-on-rails-file-upload.html

I have two tables:

Dog
- name
- weight
...

Picture
- url
- dog_id

Dog has many Picture
Picture belong to Dog

In picture_controller I just call a method 'upload' of the model at
creation:

def create
  @picture = Picture.new(params[:picture])
  post = Picture.upload(params[:picture],@picture)
  respond_to do |format|
  if @picture.save
    format.html { redirect_to(@picture, :notice => 'Created!') }
    format.xml { render :xml => @picture, :status => :created,
:location => @picture }
  else
    format.html { render :action => "new" }
    format.xml { render :xml => @picture.errors, :status =>
:unprocessable_entity }
      end
    end
  end

The method code is:

class Picture < ActiveRecord::Base
  belongs_to :dog
  def self.upload(data,insert)
    fullname = data['url'].original_filename
    name = File.basename(fullname)
    name.sub(/[^\w\.\-]/,'_')
    insert.url = name
    directory = "/public/images"
    path = File.join(directory, insert.url)
    File.open(path, "wb") { |f| f.write(data['url'].read) }
  end
end

And the view:

<h1>New picture</h1>
<% form_for(@picture, :html => {:multipart => true}) do |f| %>
  <%= f.error_messages %>
  <p>
    <%= f.label :url %><br />
    <%= f.file_field :url %>
  </p>
  <p>
    <%= f.label :perro_id %><br />
    <%= f.text_field :perro_id %>
  </p>
  <p>
    <%= f.submit 'Create' %>
  </p>
<% end %>
When I push button Create, the log shows:

Processing PicturesController#create (for 83.44.0.216 at 2010-12-11
21:16:49) [POST]
  Parameters: {"commit"=>"Create",
"authenticity_token"=>"oZTzcJa99hTRHHl9hGSTEG70cbLITHt58wkNZIMlFo4=",
"picture"=>"perro_id"=>"",
"url"=>#<File:/tmp/RackMultipart20101211-28729-1ev718o-0>}}

Errno::ENOENT (No such file or directory - /public/prueba):
  app/models/picture.rb:26:in `initialize'
  app/models/picture.rb:26:in `open'

If you have read to this point... I can only say thanks!