Amazing work you did there lossendae.
I’ve installed it and its superb. Found it easier to use for simple cases than the Gallery’s galleryitem TV
I would love to contribute adding some buttons in the thumb window as ’Maintain aspect ratio’.
This version posted is the last version of it? Do you have it at Github maybe?
github.com repository seems moved
I needed to edit some lines in the files that seems to be left there for testing

/core/components/postthumb/model/postthumb/postthumb.class.php in line 47:
//$thumb .= '&src='.urlencode('http://localhost/'.$data['src']);
$thumb .= '&src='.urlencode('/'.$data['src']);
One problem i’m having now is the final preview in the manager: the src attribute in the URL seems to be referencing always the "manager root" instead of the site root and never finds the file. the tv comes out good in the front end. Attached is a printscreen the file ULR requested after hitting the Validate this Thumb button in the thumb window.
this is the output:
<div class="crop_tpl">
<h4>Your current thumbnail:</h4>
<hr class="sep_crop">
<img src="/connectors/system/phpthumb.php?src=assets/components/gallery/files/10/75.JPG&sx=3&sy=147&sw=532&sh=191&iar=1&q=80&f=jpg&w=420&h=160" alt="Your post thumbnail" class="thumb_pw">
</div>
I know its in alpha state but love to contribute for it.