That's where its at. As an artist your work just gets spread around those wallpaper sites without proper credits/links and not asking for permission. I'll only share 640px art with a big watermark from now on. https://wallhere.com/en/tag/553911
Man, what a crap. And usually you can´t do anything or not that much against such things. The resolution easily allows people to print your work. 
Smaller res. and watermark sounds like a good idea to do from now on!! Feel sorry for you
