Itīs always a good idea to cite the source of your images just in case something like this happens.  Or, if you have purchased the image in question just provide details of the website or disk that you got it from.

As it happens, as has been proven in the past, sometimes MES images are sometimes found elsewhere (because Mark has to get them from somewhere) and those images are not illegal but unless you cite your source and/or provide proof of purchase it will be difficult to tell and Mark is very particular about people using images that he himself has purchased/paid the licence for.