Update class.image.php

header seems to be the big problem by generation on the fly.
stored icos working fine.

2 - Cannot modify header information - headers already sent by (output started at /var/www/domain/extensions/jit_image_manipulation/lib/class.image.php:447) in file /var/www/domain/extensions/jit_image_manipulation/lib/class.image.php on line 187