Ok so the console is showing the 404 image errors as you can see below:
zonsondergang.jpg:1 GET http://kbomalden.nl/images/slider/gemeentehuis.jpgimages/slider/zonsondergang.jpg 404 (Not Found)
regenboog.jpg:1 GET http://kbomalden.nl/images/slider/gemeentehuis.jpgimages/slider/regenboog.jpg 404 (Not Found)
I'm not sure why as I can see you have the image URLs correct it seems in the site source.
If you can PM me Joomla login details I can dig into this for you.
Thanks
zonsondergang.jpg:1 GET http://kbomalden.nl/images/slider/gemeentehuis.jpgimages/slider/zonsondergang.jpg 404 (Not Found)
regenboog.jpg:1 GET http://kbomalden.nl/images/slider/gemeentehuis.jpgimages/slider/regenboog.jpg 404 (Not Found)
This was not right I changed this already.
I have sent you a PM
Thanks