That cryptic picture was the clue.
The Russo Brothers lied to Marvel fans for months about the title for the upcoming Avengers: End Game movie. Way back in May, Joe and Anthony Russo confirmed that the title for Avengers 4 was not spoken in the movie. The problem is, Doctor Strange does utter "End Game" right before he dies. Fans had already guessed the name within days of Infinity War's release, and it's possible that the Russo brothers were doing their best to try to confuse everyone. They did drop off a clue that hinted at the title early in the Fall though.
Remember the picture from the set of Avengers 4 with Joe Russo sitting amongst props? It appears that the title was very carefully hidden in the picture after all. Some letters are easy to discern, like the first "E" and "N," while others seem a little far-fetched. Still, someone did guess the title correctly at that time as well.
Tony Stark also name drops the movie title in Avengers Age of Ultron. “A hostile alien army came charging through a hole in space," be exclaims. "We’re standing three hundred feet below it. We’re the Avengers. We can bust arms dealers all the live long day but that, up there, that’s the endgame."