attachment_fu, minimum width and height, how?

Hi guys, i'm using attachment_fu for uploading images, but i've to check
that the width and the height are at least 200px, how can i do this?
I've tried to do a validate

errors.add(:width, '') if self.parent_id.nil? and self.width < 200

But this doesn't work, both the width and height are nil, so i think
that them are set after the validation.
Any idea?