image size on model

Signed-off-by: Matheus Agio Nerone <man13@inf.ufpr.br>

app/models/carousel.rb

 class Carousel < ActiveRecord::Base
   has_attached_file :image, styles: {
-    larger: "300x300>"
+    larger: "300x300>",
+    thumbnail: "100x100>"
     }, default_url: "/images/:style/missing.png"
   validates_attachment_content_type :image, content_type: /\Aimage\/.*\Z/
 end