Hi,
The simplest would be to download the image
www.greenfirezuidlaren.nl/media/com_hika...ages/mail/header.png
modify it on your local computer and reupload it via FTP.
However, this means that each time you update your HikaShop, you'll loose the image and you'll have reupload it.
A nicer solution would be to upload the image with another name. Then, you could edit the email via the menu System>Emails. There, edit the template. That will open a popup with the HTML of the template. There, search for that image path ( media/com_hikashop/images/mail/header.png ) and change it to the path of your new image and save the modification and the email.
That way, you'll keep your change when you update HikaShop in the future.