Official Crossovers include images and fanart of moments that involve a REAL company's crossover of various characters in the following formats (and more): Games, comics, television, movies, books, etc.
Examples that I can think of are Smash Bros., Kingdom Hearts, Jimmy-Timmy Power Hour, more recently Steven Universe and Uncle Grandpa.
Also relating to internet celebrities (e.g. Markiplier, Nostalgia Critic), crossovers between said celebrities and also anything they've talked about and/or reviewed (be it games, cartoons, etc.) would count as "official".
Anything else falls under Regular Crossovers or other better-fitting folders.