Scope Find On Non ActiveRecord Models

I have the following setup

class User < MySuperClass #not an AR model
  attr_accessor :id, :name
end

class Picture < AR::Base
end

Picture's db table looks something like
-id
-user_id
-size

I want to be able to do @user.pictures.find(:all, :conditions => 'size

100') and other scoped find calls for pictures. Is there any plugin

or other technique I can use to achieve this sort of coding style? If
the classes were both AR models this would be as simple as
establishing an association proxy but since User is not an AR model I
don't know how to create scope for the find.

Thank you,
Matt Margolis