For your first wallpaper, this is pretty okay! But there are several improvements that I'd like to talk about.
The posing isn't the best. It makes the character look kinda lifeless. Well, it is a skeleton, but whatever. Add more bend to the arms and body, and make the character do something other than just sitting there. The shadows don't look that great to me, especially this:
I haven't used Mine-imator in a while, but find a way to make the shadows better looking, I think those are in the settings and in the rendering tab. Add some more decoration and props in the room. Stuffed animals, a desk, a lamp, a book, it helps make the wallpaper look better and a little more lively. The watermark you made in Mine-imator isn't that great, and it messes with the shadows. Instead you should export the wallpaper without the watermark first, and then add it in with something like paint.net or Gimp, maybe even Photoshop if you have that.