Sending an invoice is one option that is available to you. However, before doing this, you should consider the fact of the copyright infringement itself, and whether the infringer should have to pay for the infringement in addition to paying for the use of the photo. You may wish to increase the amount of the invoice to account for the infringement. You also may want to consider whether you want to file a copyright infringement lawsuit or send a demand letter instead of sending an invoice, particularly if you registered your copyright before the infringement and thus can recover attorney’s fees and statutory damages. If you send an invoice and it is paid, you may find that you have settled a valuable claim for too little.