Add image model spec.

spec/models/image_spec.rb

@@ -1,5 +1,8 @@
 require 'spec_helper'
 
 describe Image do
-  pending "add some examples to (or delete) #{__FILE__}"
+  it 'should not be valid without an image' do
+    image = Image.new
+    expect(image).not_to be_valid
+  end
 end