First of all, congrats on getting to where you are right now. You have layering figured out which is great, and it's good that you know how to cut yourself out from a background, and able to play with different backgrounds.
I think the next thing to study is visual story telling. See if you can tell the story of what's going on WITHOUT text. If you do need text, I try my best to avoid using any more than 4 words, but I try to not even use more than 2 words if I don't need to.
I'd also study "composition", studying coposition and visual heirachy will help point out the most important things in your thumbnail, for example in this thumbnail, you could make the phone and text screen SIGNIFICANTLY larger, and yourself larger as well. The emojis and "omg" indicate something interesting is going on, so that alone would be more interesting to the viewer than laying everything on the table. "he tried it! so i played the game better" really doesn't tell me a whole lot more about what's going on, than having the phone it'self, but the phone with text kinda indicates there's a story going on, and you won't even need the title on the thumbnail to point that out.
One last point on words on a thumbnail. Your TITLE area is more important for the title, having it on your thumbnail as well kinda just duplicates it. I'd see the thumbnail without text, and then the title would be under it anyway to give more context.
So you're on the right path, it's now just time to fine tune it all
but great work on your layering.