Original art by Tim015 ( http://tim015.deviantart.com/ ) Go check out his profile!
Easy! You just need to paste this in your "logos" folder:
steam\SteamApps*your username*\team fortress 2\tf\materials\vgui\logos
Then go into tf2=>settings=>multiplayer=>import spray=>and select the file.
And don't forget: Rainbow Dash is best spray.