Kareha Imageboard Thumbnails - How do they work?

Hi

I'd like to use "smart" thumbnails, but when I turn off stupid thumbnailing, image posting doesn't work at all anymore.

use constant STUPID_THUMBNAILING => 0;			# Bypass thumbnailing code and just use HTML to resize the image. STUPID, wastes bandwidth. (1: enable, 0: disable)

Do I need to change something else or does it just not work on OpenWrt?