resizing images in rails?

Hi,

    how to resize the images in rails app?
    i want 400X300 size images to display in my app.But if user enter
600X800 how to resize the image without effect on clarity?

  now i am using this code

     image =
Magick::Image.read("#{RAILS_ROOT}/public/gallery/#{review_img}").first
     selected_image = image.crop_resized(400,400)
     selected_image.write("#{RAILS_ROOT}/public/gallery/#{review_img}")

   But it extract height and width.i dont want to extract only resize
the image.
   any suggestion are appreciated.

thanks,
ravi.

==model (using attachment_fu)

class Picture < ActiveRecord::Base
  belongs_to :user

  has_attachment :content_type => :image,
                 :processor => 'image_science',
                 :storage => :file_system,
                 :path_prefix => File.join('public', 'images',
'pictures', ''),
                 :resize_to => '400',
                 :thumbnails => { :thumb => [150, 203]}

  validates_as_attachment
end

will resize the image when created.