Like Martina below, I was contacted by these individuals about a stock image that I legitimately purchased from another well known stock company. They were claiming that I had somehow fraudulently got it from them instead and demanding payment! Given that this image is available on several different stock sites, I conclude that this is a scam designed to extort money from site owners who don't keep track of where they bought their images from. Luckily we do keep all licensing details so they didn't succeed in scamming us. Avoid them at all costs!
