Like AngeKrystaleen said: when you want to ask about a work being deleted, you post in the Deletion Appeals forum. There, the staff member who deleted your artwork will
explain why they did so. The Sandbox is for ongoing works that people want to get feedback on. The difference between
the two is that only staff members may post in the Deletion Appeals forum (for obvious reasons), while everyone can post
their opinions in The Sandbox. Wanted to clear that up before going any further :)
As for the Pixiv issue, we no longer allow artwork from it to be used or submitted on this site unless it's with
the explicit permission of the artwork's creator. That is, we must
know you got 100% permission to use their artwork (mention it in the description, link to the artist's artwork -
you can even include a screenshot of the artist giving you permission to use their work, just to be on the safe side). I
took a look at your wallpaper and nowhere in the description does it say that you have permission to use that artwork.
So, in order to be able to upload your wall again, you would have to provide proof that the artist gave you permission
to use their artwork in it.
See this thread: Work from Pixiv no longer allowed
for more details (it's in the News forum).