Hi,
I've checked your website and the thumbnails are properly generated.
The main image of that category is:
www.jvk-start4.nl/media/com_hikashop/upload/volvo.jpg
So it's normal that the thumbnail generated is this one.
And on the backend, you get another thumbnail because the thumbnails are generated in the media/com_hikashop/upload/thumbnail_xxxxyyy folders that you didn't delete.
Delete these folders and you'll get the same thumbnail are the frontend because the problem is with the origin image which is not correct, not with HikaShop.