can't generate a thumbnail of an image using carrierwave

my plan is i want to create a model only for images and use that model to create profile pictures, gallery etc..

so, i have created a separate model for images and i was able to store images in display those images. but, they are too big and so i want to create a thumbnail version. my configuration is

class Image < ActiveRecord::Base
 mount_uploader :avatar, AvatarUploader
end

controller:

class ImagesController < ApplicationController
   def new
     @image_upload=Image.new
   end

   def create
     @image_upload=Image.create(uploading_image)
     if @image_upload.save
      redirect_to '/users'
     end
  end

  def uploading_image
     params.require(:image).permit(:avatar)
  end
 end

uploader:

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

   def store_dir
      "uploads/#{model.class.to_s.underscore}...(default store only)"        
   end


   version :thumb do
     process :resize_to_limit => [50, 50]
   end

user controller:

def index
   @user_profile=Profile.find(1)
   @imagefile=Image.first
end

users/index.html.erb:

 <%= image_tag @imagefile.avatar.to_s %> #This gives me the whole image.

<%= image_tag @imagefile.image_url(:thumb).to_s %> #says undefined method `image_url' for #<Image:0x007f6cd0538010>

ps: i can see a thumbnail version in the folder

It would be so great, if anyone could help me out.