And generally, the pictures nominated within a month, were also posted that month. But if you happen across a "wow" shot from 6 weeks ago, don't hesitate to nominate it. In December, I nominated a picture out of the series of the Cobra. That was not highlighted in December, but someone else nominated another one from that same post in January and that one actually was picked.
If in doubt, nominate

.
Or six months, or more! The whole purpose of the POTM is to highlight some of the quality aviation photography we have posted on Fence Check, it's not a competition and every shot chosen is equal in stature. The only reason we have one shot highlighted larger each month is purely because of the way the forum software is set up, every single shot chosen is neither better or worse.
So if you see anything old that jumps out at you whilst scrolling around then feel free to nominate it and it'll be considered.