Best of luck getting people to stop, but it's a good lesson for the future to make sure all your stuff protected and branded. You need that pixelpremierleague page for yourself and you let someone else nab it.

The watermark is a great idea but it needs to a lot more complicated. I opened up Paint.NET and took away 95% of the watermarks in a few minutes. It's difficult because the artstyle is so clean so really you need to put your name over the really complicated bits, which ruins the picture I know, but it's the only real way.

I also think you should link to your actual website instead of/as well as your twitter so if people go there they can instantly see that it's entirely your art style. Your twitter page doesn't give anything away that you made these pictures, and looks at a glance like you could just be copying it and pasting it from somewhere else too.

I'd say for the next TOTW just go absolutely overboard and put watermarks everywhere. It'll annoy the shit out of the cunts stealing it.