Cropping images with RMagick after uploading with acts_as_attachment


I'm using acts_as_attachment to create thumbnails in a filesystem (no problem there), but I'm having a rough time figuring out how modify the image to crop it, and then put it back into the filesystem.

I think that I can pull the image OK and modify the Image OK with RMagick.

So really I just need a pointer on putting it back into the filesystem as a modified thumbnail, but a semi-complete example would be a bonus.