My first guess would be something wrong with your server...
Second guess would be you didn't install 4images properly (i.e. permissions are not set properly and/or some files uploaded to the server with a wrong method (ASCII vs BINARY)
Try reupload 4images and double check all permissions or you can try use re-captcha instead:
http://www.4homepages.de/forum/index.php?topic=22881.0