avatar.rb

class Avatar < ActiveRecord::Base
  mount_uploader :avatar_image, AvatarImageUploader  # **must** specify class of uploader, otherwise base uploader is used, and no 
  
  # ...
end

avatar_image_uploader.rb

class AvatarImageUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

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

  version :print do
    version :thumb    { process :resize_to_fit => [32, 32] }
    version :preview  { process :resize_to_fit => [256, 256] }
    version :full     { process :resize_to_fit => [2048, 2048] }
  end

  version :web do
    version :thumb    { process :resize_to_fit => [32, 32] }
    version :preview  { process :resize_to_fit => [128, 128] }
    version :full     { process :resize_to_fit => [1024, 768] }
  end

  # ...
end

carrierwave.rb

# config/initializers/carrierwave.rb

CarrierWave.configure do |config|
  config.fog_credentials = {
    # Configuration for Amazon S3 should be made available through an Environment variable.
    # For local installations, export the env variable through the shell OR
    # if using Passenger, set an Apache environment variable.
    #
    # In Heroku, follow http://devcenter.heroku.com/articles/config-vars
    #
    # $ heroku config:add S3_KEY=your_s3_access_key S3_SECRET=your_s3_secret S3_REGION=eu-west-1 S3_ASSET_URL=http://assets.example.com/ S3_BUCKET_NAME=s3_bucket/folder

    # Configuration for Amazon S3
    :provider              => 'AWS',
    :aws_access_key_id     => ENV['S3_KEY'],
    :aws_secret_access_key => ENV['S3_SECRET'],
    :region                => ENV['S3_REGION']
  }

  # For testing, upload files to local `tmp` folder.
  if Rails.env.test? || Rails.env.cucumber?
    config.storage = :file
    config.enable_processing = false
    config.root = "#{Rails.root}/tmp"
  else
    config.storage = :fog
  end

  config.cache_dir = "#{Rails.root}/tmp/uploads"                  # To let CarrierWave work on heroku

  config.fog_directory    = ENV['S3_BUCKET_NAME']
  config.s3_access_policy = :public_read                          # Generate http:// urls. Defaults to :authenticated_read (https://)
  config.fog_host         = "#{ENV['S3_ASSET_URL']}/#{ENV['S3_BUCKET_NAME']}"
end

Gemfile

# ...

gem 'carrierwave'
gem 'fog', '~> 1.0.0' # Need to specify version, as carrierwave references older (0.9.0) which doesn't allow configuration of Rackspace UK Auth URL

show.html.erb

<!-- Nested Versions -->
<%= image_tag avatar.avatar_image.url(:web, :preview) %>
<%= image_tag avatar.avatar_image.url(:print, :full) %>
<%= image_tag avatar.avatar_image.url(:web, :thumb) %>