I can understand being miffed at your image being used if another person explicitly takes credit for it. But I’m less certain how it harms you if someone uses in it such a way that they aren’t claiming authorship (and in fact are linking back to your site).

Presumably it isn’t hurting you financially since you aren’t selling the image. Perhaps you wish the image to be unique to your site, but that’s really a pretty abstract idea since it’s probably pretty rare that someone who visits your site also happens to visit the site of the person who is using your image. Those who do visit both sites are probably doing so because they’re following the image’s link (so that the person borrowing the image is actually promoting your site).