Hi,
The message you have in the backend is not a real problem.
Because the field is required and HikaShop try to store the default value when you save the field, the message is displayed.
But it does not affect the rest of the save for the custom field.
So if you want to save as the default value an empty value, you have to remove the "required", save the custom field (with an empty value) and then re-put the default value.
We will see to remove this message in a future release and improve the save of a empty default value even if the field have the "required" set.
For the front-end part, I will require some access (backend & ftp) in order to make some debug ; to understand the source of the problem.
But the fact that there is a default image is problematic for the process of the custom field.
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.