I had something like this a little bit ago, my friend saw that on a Civilization 5 mod, someone was using one of my maps as the thumbnail, banner, and the main banner for the mod. Of course, this was extremely flattering, and so I cut him some slack, but what angered me is that there was no mention of me, who made the map, or any link back to the original Cartographers Guild or Deviant Art post, (This was a version without labels, when I was in the WIP.) I couldn't really figure out what to do, and didn't know any of his specifications to ask him to put my name on their, but if you didn't give them permission, and they aren't linking back, then you should talk to them, even while I couldnt