Rails: How to run before_update only for one changed attribute?

In my model Shop I'm saving image url in logo_ori and use that to make thumbnails using before_update.

# shop.rb
before_update :run_blitline_job

private

def run_blitline_job
  # uses logo_ori to make thumbnails
end

However I found out that when I'm saving other attributes (eg: editing shop's profile in a form) it also runs before_update. How do I confine its execution when only logo_ori is saved?

I've tried this :

before_update :run_blitline_job, :if => :logo_ori?

but it still runs before_update if I already have logo_ori saved earlier.