There is an easy way to override this. All you have to do is set the style in the original:
class User < ActiveRecord::Base has_attached_file :avatar, :styles => { :original => "500x500>" } end
It will not save the original and will not accept the entire input image and will not change it in accordance with your specifications. Then, when you want to access it, you do not need to specify a style.
image_tag @user.avatar
Instead:
image_tag @user.avatar(:thumbnail)
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