attachmnet_fu can't keep original file image!

I am currently trying to create an application on RoR and I am using
attachmne_fu plugin in order to handle image uploads.
I have also install ImageScience to create some thumbnails for the
images I upload

The problem is that I can upload images, but it resizes the original
image to the thumbnail version. I don't know why.
For example if I upload an image say IMG_2455.jpg, in the directory
0000/0002 I have 2 files:
a) IMG_2455.jpg
b) IMG_2455_thumb.jpg

but the original image is also resized! so i have two images of 125x83
dimensions.
How can I keep the original image plus the thumbnail version ?
Here is my code in my model

class Photo < ActiveRecord::Base

  belongs_to :user

  has_attachment :content_type => :image,
                   :storage => :file_system,
                   :size => 1.byte..12.megabytes,
                   :thumbnails => { :thumb => '125x83>' },
                   :path_prefix => "/public/photos/"

    validates_as_attachment

end

Any suggestions ??

It may have nothing to do with your situation, but I once had a
similar experince when I forgot to install image magick...

but I think Ar's advice will probably work

Cheers

Richard,