Hi,
After you get that error, go in the HikaShop configuration page, and in the files section, open the "payment log file". That will open a popup. And at the end of the popup with all the log of HikaShop. And at the end, you should find such message:
Image xxx could not be opened when trying to generate its thumbnail
The xxx will be the file name of the image which generated that error when HikaShop tried to generate the thumbnail for it.
Knowing what you have there should help us better understand the situation you're in with that error message.