I don't like how post_param_image and resize_rep_image and check_form_field_image are defined in themes2.php, should be images2.php.

Note that this change is a big one. A lot of code has been restructured. Thumbnails are now in most cases something that are generated on the fly - with the exceptions of videos and attachments and the media system in general, which support explicit thumbnails as they may not be auto-generatable.
The thumbnail editor addon has gone because you no longer have one thumbnail for resources but potentially many generated on-the-fly.

A clear distinction is maintained between thumbnails ("an alternate representation of a visual resource") and representative images (just any image the webmaster has chosen to represent something that is not intrinsically visual).

GD is now strictly required. It was always in the requirements but we did still unofficially support not having it. Not anymore.