banner.rb

class Banner < ActiveRecord::Base
  attr_accessor :image_width, :image_height
  mount_uploader :image, ImageUploader
  
  validate :check_dimensions
  
  def check_dimensions
    if !self.image_cache.nil?
      errors.add :image, "Dimensions of uploaded avatar should be not less than 150x150 pixels." if self.upload_width < 150 || self.upload_height < 150
    end
  end

end

image_uploader.rb

# encoding: utf-8

class ImageUploader < CarrierWave::Uploader::Base

  include CarrierWave::MiniMagick

  # Choose what kind of storage to use for this uploader:
  storage :file

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  def default_url
    "/uploads/missing/#{model.class.to_s.underscore}/#{version_name}.png"
  end
  
  # for image size validation
  # fetching dimensions in uploader, validating it in model
  before :cache, :capture_size_before_cache # callback, example here: http://goo.gl/9VGHI
  def capture_size_before_cache(new_file) 
    if model.upload_width.nil? || model.upload_height.nil?
      model.upload_width, model.upload_height = `identify -format "%wx %h" #{new_file.path}`.split(/x/).map { |dim| dim.to_i }
    end
  end
  
  # resizing uploads
  
  process :resize_to_fill => [148, 148]

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_white_list
    %w(jpg jpeg png)
  end

end