I'll try to get straight to the point...

Good things:
- Perspective
- Charcter's vectoring, nicely done, especially on her dress.
- Composition. It's well placed and the scene doesn't feel too busy or too empty

Not so good things:
- Lighting/Shading of the scene. It's completely missing, the colors of the room and of the objects are completely flat, same for the girl. It's hard to tell what is the main source of light and where is placed. There's also total lack of shadows on the objects and on the whole scene as well. Which makes the scene sort of feel "incomplete", somehow...
- The background on the outside. First, off the perspective of the tree, compared to the scene inside, isn't exactly correct. Second thing, it seems to be a stock-photo and the tree itself shows much more details than the elements inside the room, which is not normal since according to the usual perspective rules, the farther elements the lesser the details. I'd almost suggest you to completely redo the outside scene and scrap that tree/stock photo. For example you could draw a beach/seaside on the outside, seen from the window as if the room where she is was on the second floor of a house, following the same perspective you adopted for the inside scene itself.

And that's it. It's definetely a good display of effort and it's absolutely worth a praise for it, it just needs to be furtherly polished and a bit more work on. If you don't plan to work on it any further (which I can perfectly understand, after having worked 3 months on it), you can always keep in mind those suggestions for any future wallpaper that might present a similar issue. :)

Panels? O_o
No wonder I mistook that for a window though, since it did give that illusion, probably because the lighting/shading of the room is a bit incomplete. Oh and it really doesn't matter if you can't do more, as long as you know what was off as you'll gain experience your next works are bound to be better. :)