I see you worked out cropping it. That's the main thing. That's all I was going to do but you did it.
You could put that onto a shape so that it's a horizontal orientation and fill in color on the sides so the mask is a color that matches the painting but see what works.
You're not meant to buy it yourself. You advertise them on twitter or Instagram. And once the art is ready whether it's a coffee mug or a sweatshirt I think it's the same. I wouldn't buy and try to sell them though. They sell them and you get paid. Yes, make your online shop and then you can put it on twitter with hashtags like #mask and #dogs #art #portraits, etc.
You could buy a mask or a coffee cup or whatever for photos and your twitter or instagram picture.
Have fun. Good luck.
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This is the idea of filling in the color. I just put the original in layer 2, sampled the green and filled layer 1 with it, then merged down.