Add image fixture and validation.

spec/factories/photos.rb

@@ -4,5 +4,6 @@ FactoryGirl.define do
   factory :photo do
     title "A Photo"
     description "A lovely photo of a tree"
+    image Rails.root.join('spec/fixtures/photo.jpg')
   end
 end

spec/models/photo_spec.rb

@@ -3,6 +3,8 @@ require 'spec_helper'
 describe Photo do
   it { should have_and_belong_to_many(:categories) }
 
+  it { should validate_presence_of(:image) }
+
   let(:photo) { create(:photo) }
 
   it "logs a view" do