Hi there,
Ok, I've tried my best but it's not possible in current phones. In exchange I added as many paints as possible and picking up as many ideas as I could from the forum. There will be over 50 decals on the next update.
But...
I can't add a custom paint for everyone. It would kill the phones and lag the servers too much. To explain why, imagine you created your own insignia on the plane. Now that's yours, it's not distributed with the game.
When people joins a game, 8 people (including you) will have to request that graphic to the server. Download it, parse it and convert it to an internal format for the game. That takes some time and delays the connection. If you have 8 people each with their own insignia... that's each of them downloading 8 graphics... that's 8x8 = 64 doing the download, parse and internal conversion. And this is only for 1 room. Imagine now all players in all rooms doing this upon connection... you think lag is bad? Don't ask how it would be
This is ok on modern phones/game consoles/pcs. But it sucks for most phones.
In short, for now... it cannot be done. Hopefully next year when the average phone is a Samsung galaxy 4. Until then, we have to stick with the ones we added to the game manually.