how to write this where paperclipe understands

I can do this command to resize an image to fit a specific size on the command line with Imagemagick. How do I tell paperclip can do the same开发者_运维技巧 when I upload an image:

convert Bisiye2.jpg -thumbnail '150x150^' -gravity center -extent 150x150 Bisiye2_tofit.jpg

---

  has_attached_file :image, :styles => { :thumb => "150x150>" }, 
    :convert_options => {:thumb => "-gravity center -extent 150x150"}