Three modules that offer nice thumbnail cropping for ProcessWire. Fully compatible with normal image field.
Just add files and folders to your site/modules/ folder and install one of the modules (it installs others). After that create new field or edit existing image field and make it use fieldtype "CropImage".
You can change the settings for each field in the Setup->Files->CropImageField Input tab:
Usage on templates
FieldtypeCropImage adds getThumb-method to Pageimage. getThumb returns url to the thumbnail you have asked for, ie:
If you haven't cropped that thumbnail yet, this module provides default cropping from center
Developed by Antti Peisa (twitter.com/apeisa)
Sponsored by http://stillmovingdesign.com.au/ (thanks Martin!)