i did something with the captcha-file ... there are now real background images...let's see if it works
Code:
source:
http://www.phpbb.com/mods/db/index.php?i=misc&mode=display&contrib_id=10865
...just replaced the $img = imagecreatetruecolour(); for the capturepic with $img = imagecreatefromjpeg();
but we should have a look for cooler backgrounds
--> jpeg/jpg 360*96