If you are looking for a WordPress plugin that allows the user to define where the image will be cropped for a given thumbnail size, you might try these plugins.
Post Thumbnail Editor
To meet the needs of themes where the post-thumbnails have random and capricious sizes (which causes wordpress to crop images simply from the middle (either chopping off the top and bottom or chopping off the sides), this plugin attempts to give the users an interface to manually fix those thumbnail and random images.
More info and download here:http://wordpress.org/plugins/post-thumbnail-editor/screenshots/
“Crop Thumbnails” made it easy to get exactly that specific image-detail you want to show. Crop your images the simple way.
More info and download here: http://wordpress.org/plugins/crop-thumbnails/
Easy Image Crop
Adds super intuitive user interface for WYSIWYG image cropping. This image cropping plugin also supports custom image sizes and helps you add, manage.
More info and download here: http://wordpress.org/plugins/easy-image-crop/