avatar_uploader.rb

class AvatarUploader < CarrierWave::Uploader::Base
  include CarrierWave::RMagick
  storage :file

  def store_dir
    "assets/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  version :thumb do
    process :resize_to_fill => [60, 60]
  end

  version :medium do
    process :resize_to_fit => [200, 120]
  end

  def extension_white_list
   %w(jpg jpeg gif png)
  end
end

profile.rb

class Profile < ActiveRecord::Base

  mount_uploader :avatar, AvatarUploader
  
  # Usual stuff...

end