fix some glitches in gravatar image url generation

always use proper size value
do not show alt text when avatar image not loaded

app/decorators/commit_decorator.rb

@@ -76,7 +76,7 @@ class CommitDecorator < ApplicationDecorator
     source_name = send "#{options[:source]}_name".to_sym
     source_email = send "#{options[:source]}_email".to_sym
     text = if options[:avatar]
-            avatar = h.image_tag h.gravatar_icon(source_email, options[:size]), class: "avatar #{"s#{options[:size]}" if options[:size]}", width: options[:size]
+            avatar = h.image_tag h.gravatar_icon(source_email, options[:size]), class: "avatar #{"s#{options[:size]}" if options[:size]}", width: options[:size], alt: ""
             %Q{#{avatar} <span class="commit-#{options[:source]}-name">#{source_name}</span>}
           else
             source_name

app/helpers/application_helper.rb

@@ -30,7 +30,9 @@ module ApplicationHelper
     args.any? { |v| v.to_s.downcase == action_name }
   end
 
-  def gravatar_icon(user_email = '', size = 40)
+  def gravatar_icon(user_email = '', size = nil)
+    size = 40 if size.nil? || size <= 0
+
     if Gitlab.config.disable_gravatar? || user_email.blank?
       'no_avatar.png'
     else