You are right you will need to add upc/ean code to your listing on Amazon. You can try to contact Amazon customer care and tell them about the nature of your products and try to convince them to exempt your paintings from UPC Code requirement.
Check out this article can help you to understand Amazon’s UPC compliance guidelines.
I co-own Nationwide Barcode. We are a UPC barcode reseller and if Amazon will not give you an exemption, check out my site. We are an inexpensive solution for getting barcodes legally and inexpensively. We are a Laurer approved site (he is the inventor of the UPC barcode) and have a BBB-A Rating. Let me know if you have any questions.