If you have installed Statamic on Windows and find image thumbnails not working, the issue for me was that my PHP installation did not have EXIF extensions enabled. To solve this problem open your php.ini file, and make sure that the following two extensions are enabled:
php_mbstring.dll (this one probably is) php_exif.dll (this one wasn't for me)
That's it. Re-start your server and things should work.