admin-projects.rb

ActiveAdmin.register Project do

  # Don't forget to add the image attribute (here thumbnails) to permitted_params
  controller do
    def permitted_params
      params.permit project: [:title, :summary, :description, :thumbnail, :date, :url, :client_list, :technology_list, :type_list]
    end
  end

  form do |f|
    f.inputs "Project Details" do
      f.input :title
      f.input :thumbnail, :as => :file, :hint => f.template.image_tag(f.object.thumbnail.url(:thumb))
      # Will preview the image when the object is edited
    end
    f.actions
  end

  show do |ad|
      attributes_table do
        row :title
        row :thumbnail do
          image_tag(ad.thumbnail.url(:thumb))
        end
        # Will display the image on show object page
      end
    end
end

project.rb

class Project < ActiveRecord::Base
  has_attached_file :thumbnail, :styles => { :medium => "300x300#", :thumb => "200x200#" }
  
  # 200x200# crop the image
  # 200x200> scale the image to 200px wide at maximum keeping aspect ratio
end