Hello, Jan-Lukas;
sorry you have a problem.
I went to your website and viewed each time the drilling eye and the 404 error icon.
Could you please go to your PhpMyAdmin interface, and check what is the value in:
your_prefix_settings (eg: 4images_settings).
and then the entry "allowed_mediatypes".
It should be something like this:
jpg,gif,png,aif,au,avi,mid,mov,mp3,mpg,swf,wav,ra,rm,zip,pdf
Hope it could help you a little.
Regards.
manurom