diff --git a/app/models/screenshot.rb b/app/models/screenshot.rb
index 9121090..a9b4c2c 100644
--- a/app/models/screenshot.rb
+++ b/app/models/screenshot.rb
@@ -14,7 +14,9 @@ class Screenshot < ActiveRecord::Base
   validates_with AttachmentSizeValidator, :attributes => :image, :less_than => 5.megabytes
   validate :validate_image_is_unique
 
-  # Validator that checks if the image has already been uploaded
+  # Validator that checks if the image has already been uploaded.
+  # A generic uniqueness validator does not work because it would attribute
+  # the error to the :image_fingerprint field and not the actual :image field.
   def validate_image_is_unique
     # Look for images with the same checksum / image_fingerprint
     if Screenshot.find_by(image_fingerprint: image_fingerprint)