=Paperclip
Paperclip is intended as an easy file attachment library for ActiveRecord. The intent behind it was to keep setup as 
easy as possible and to treat files as much like other attributes as possible. This means they aren't saved to their 
final locations on disk, nor are they deleted if set to nil, until ActiveRecord::Base#save is called. It manages 
validations based on size and presence, if required. It can transform its assigned image into thumbnails if needed, and 
the prerequisites are as simple as installing ImageMagick (which, for most modern Unix-based systems, is as easy as 
installing the right packages). Attached files are saved to the filesystem and referenced in the browser by an easily 
understandable specification, which has sensible and useful defaults.
See the documentation for the +has_attached_file+ method for options.
==Usage
In your model:
  class User < ActiveRecord::Base
    has_attached_file :avatar, :styles => { :medium => "300x300>", :thumb => "100x100>" }
  end
In your migrations:
  class AddAvatarColumsToUser < ActiveRecord::Migration
    def self.up
      add_column :users, :avatar_file_name, :string
      add_column :users, :avatar_content_type, :string
      add_column :users, :avatar_file_size, :integer
    end
    def self.down
      remove_column :users, :avatar_file_name
      remove_column :users, :avatar_content_type
      remove_column :users, :avatar_file_size
    end
  end
In your edit and new views:
  <% form_for :user, @user, :url => user_path, :html => { :multipart => true } do |form| %>
    <%= form.file_field :avatar %>
  <% end %>
In your controller:
  def create
    @user = User.create( params[:user] )
  end
In your show view:
  <%= image_tag @user.avatar.url %>
  <%= image_tag @user.avatar.url(:medium) %>
  <%= image_tag @user.avatar.url(:thumb) %>