This is not a default feature of Dokan to make the product image field as required. You will need to do some custom code to achieve this. For guidance on how to achieve it, you can follow this blog step by step.
I hope it will help you to achieve it. Let me know further if it worked for you.
for the add_filter( ‘dokan_new_product_popup_args’, ‘dokan_new_product_popup_validation_customized’, 35, 2 );, how do I change the dokan_new_product_popup_args? Because I don’t use pop up. I don’t want to use pop up. Do you know the code?