## image.rb

class Image < ActiveRecord::Base
  belongs_to :article

  has_attached_file :image, :styles => { :thumb => "100x100#", :standard => "300x200#" }
end

## images_controller.rb
class ImagesController < ApplicationController
  def new
    @article = Article.find(params[:id])
    @image = @article.images.new
  end

  def show
    @image = Image.find(params[:id])
  end

  def create
    @image = Image.new(params[:image])
    @image.user_id = session[:user_id]
    if @image.save
      flash[:notice] = "Image successfully uploaded"
      redirect_to :controller => 'articles', :action => 'show', :id => @image.article
    else
      flash.now[:error] = "Image could not be uploaded"
      render :action => 'new'
    end
  end
end
## new.html.erb

<% form_for :image, :html => { :multipart => true } do |form| -%>
     <%= form.error_messages %>
  <p><%= form.file_field :image %></p>
     <%= form.hidden_field :article_id %>
  <p><%= form.submit :create %></p>
<% end -%>