使用paperclip這個gem，參考以下鏈接：
https://github.com/thoughtbot/paperclip
Gemfile

gem "paperclip", "~> 4.2"

rails g paperclip good good_image
rake db:migrate

models/good.rb

has_attached_file :good_image, :styles => { :medium => "300x300>", :thumb => "100x100>" }, 
                  :default_url => "/images/:style/missing.png"
validates_attachment_content_type :good_image, :content_type => /\Aimage\/.*\Z/

controllers/goods_controller.rb

def good_params
      params.require(:good).permit(:name, :price, :description, :discount, :good_image)
end

views/goods/_form.html.erb

<%= form_for(@good, html: {multipart: true}) do |f| %>
  ...
  <%= f.file_field :good_image %>
  ...
<% end %>

views/goods/show.html.erb

<%= image_tag @good.good_image.url %>
<%= image_tag @good.good_image.url(:medium) %>
<%= image_tag @good.good_image.url(:thumb) %>

這樣就能簡單的調(diào)用了。。。修改missing圖片的話，可以如下：
assets/images文件夾下增加

original/missing.png
medium/missing.png
thumb/missing.png

good.rb

:default_url => ":style/missing.png"

這樣，當沒有圖片的時候，會顯示對應(yīng)的app/assets/images/:style路徑下的missing.png