A common misconception is, that in order to enforce your rights with Amazon you must have a registered copyright.
Indeed, Amazon prefers to see documentation. If you register your copyright in the US copyright registry (copyright.gov), and provide Amazon with the certificate of registration, Amazon will enforce your right immediately and remove the infringing image from your competitor’s listing.
However,… You can still fight the infringement, even without registration !
Legally, from the moment you created your image, even a simple picture, taken with your phone, your work is fully protected by copyright – even without registration. Copyright registration is voluntary, mainly for evidentiary purposes, and not mandatory.
Therefore, if you prove to Amazon, with clear and convincing evidence, that you posted the image on your website, or on your listing before the infringer did – most likely, Amazon will enforce your rights and remove your image from the infringer’s listing.
The link to place a copyright complaint with Amazon is: https://www.amazon.com/report/infringement
Be persistent and convincing and use solid proof (such as, links to your images) to prove that you were there first!
P.s. the infringing image must be identical to yours. Amazon rarely accepts infringement claims based on “similarity”.