As was mentioned elsewhere, the image used is a copy of a copy. Arrangements of 1's and 0's are not a scarce, rivalrous resource, and thus no one can place a claim of ownership on them. You cannot own an idea, as ideas aren't scarce, and thus they are excluded from property norms.

This is why so-called piracy cannot possibly be theft. Copying something and leaving the original deprives no one of anything. You are entitled to the proceeds from selling what you have (your original photograph) to someone or to a group of people. You are not entitled to compensation from a later user who has utilized a copy of your original work.

Using myself as an example, I write. I produce fiction and non-fiction. If I wanted to sell what I produced to someone, ethically I am entitled to the agreed-upon amount for the sale, and for any other arrangements for compensation I have established with the buyer. However, if someone were to copy my work and print it themselves, I am not ethically entitled to any proceeds arising from their sale. I would be well within my rights to point out that this work is not theirs and show how it isn't (reputations exist for this reason), but I have no grounds whatsoever to seek restitution.

Use without attribution is a poor practice, to be certain, and I don't agree with it. However, making the leap into theft is baseless. There's no consistent ethical foundation for it. Nothing you own has been stolen by the copy that was used, as tasteless as its use was.