It's the "Visit The Lost Continents"-poster as seen in the game (such as in Steel BLU spawn), created with pieces of cut cardboard, apart from the bottom text.
(Poster as seen in-game)
This took me around 15 hours in total between two days. I didn't have much prior experience with layered...
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble