The problem with Google Webmaster Tools is that they create and validate a robots.txt file that Google recognizes, but it does not adhere to the standard. The Allow directive is not part of the robots.txt standard. Other spiders may recognize it, but you will not find the Allow directive to be part of the standard specifications.
If you just want to block the images folder, use the following:
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin