I think that while it's a cool screencap, it really doesn't work well here. The style is just too different from your drawing style, and it seems out of place. I think there are different ways that you could work to integrate the background into this piece, for example, changing the color of the grass brush and the bottom, because right now the colors clash a little. Or, if there were more mushrooms in the background, it would tie together the mushroom that she is sitting on.
You've also put a lot of work on shading your OC, which I think brings out a lot of detail in this piece. The only critique I have for him is that his head is disproportionately big for his body. By a lot. If you put your hand to your face, with your palm of the bottom of your chin, you fingers should reach high up on your forehead. His hands in this piece would only touch to his nose. Just something to consider : )
No problem. I definitely think it's a good idea to screw around in photoshop, god knows I did it for ages to try and figure the whole thing out. There are so many awesome features and brushes, kudos to you for exploring it : )
For this, I feel like you either have to just draw everything more in the same style (draw your own background, and either stick with outlined art, or not outlined, and do something so the grass tool doesn't look so obvious), or REALLY EMBRACE the hodgepodge, somewhat collage look to the piece. Right now, it looks like you just got lazy, but from other comments, I saw you wanted to experiment with putting various styles together.
If you really want to push the collage aspect, make the background a combination of SEVERAL different game backgrounds (throw in a bit of Skyrim behind the bamboo, and maybe Mordor or something behind that, with a Mass Effect sky or something, I dunno, I'm just throwing out words), and instead of treating the head and the body differently on the guy, treat his skin differently from his leather clothes, and differently from the metal; like, different methods of putting it together ENTIRELY, maybe even scanned materials. Go CRAZY. Go big or go home!