Hi,
It is more a trick than a real process.
What I generally do is to create a folder near the "media/com_hikashop/images" one, called "media/com_hikashop/my_images".
I placed my overridden images there and each time I need to (after an update, etc), I just copy the content of "my_images" to "images" (using "cp -r").
So I have a special folder just for the images I override and I'm sure that folder is not replaced when I update HIkaShop.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.