Just a couple of observations:
The "distant" photo that doesn't meet site criteria for an individual category ought to be OK for a while, provided reasonable location/date info is given. They can quite often be identified using AIS data once the company and/or class is established - but not all members can access practical search tools for this.
Good quality mystery ships could stay longer than a year - new members are joining all the time, with different knowledge, and different resources which can solve old chestnuts. In many cases they cannot be moved to another category due to uncertainties over categorisation (eg age, tonnage, function etc). The Poirots amongst the members love to look again at "cold cases".