Bill Erickson

When you upload an image, WordPress will automatically scale/crop it to many different sizes. If you’re not happy with the auto-crops, use this plugin to upload an alternative image.
If you have images showing up in your theme already, there’s nothing you need to do after activating this plugin. All WordPress functions that provide the thumbnail should now automatically work with your override image.
This plugin will add a metabox to every post type and allow you to modify every image size (built-in and custom ones added using add_image_size). You can use two filters to change these (image_override_post_types and image_override_sizes). For examples, see the documentation.
<a href="https://github.com/billerickson/Image-Override/wiki" rel="nofollow ugc">Documentation</a>
No support will be provided by the developer

Image Override

Allows you to override WordPress' auto generated thumbnails.

sewpafly

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.

Post Thumbnail Editor

Does not work

Fed up with the lack of automated tools to properly crop and scale post thumbnails? Maybe this plugin can help.

Image Override vs Post Thumbnail Editor