How to manually fix cropped portfolio thumbnails

I am using 3:2 aspect ratio for my thumbnails in my portfolio pages, but the auto crop in many cases chops the heads off (http://www.samerkurdi.com/?page_id=889&paged=2), or doesn’t quite satisfy/center properly.

How can I manually define the crop for each thumbnail? The portfolio thumbnail sizes do not seem to show up in the media gallery.

Thanks.