Thanks for the answer.
I have already added the QR code and it works fine, but the problem is that I have 2 bank accounts (currently added as custom field).
For each invoice I choose one of those two bank accounts from the dropdown and I need the corresponding QR code to be used (depending on the bank account) on the invoice, because it will hold the bank account number which is important for the payment.
E.g.:
Bank account number for EUR - 1st QR code
Bank account number for USD - 2nd QR code
For such reason I can not simply hardcode one QR code.
When I am thinking about it there might be an option to have 2 custom designs, but like this I will be limited to 3 bank accounts.