Proposal: Allow down votes. Periodically remove the most down voted entries.
Rationale: A major problem with the photo contest is the only way to remove an entry from the list is for it to "win". With over 450 entries, it can run for 8-9 years with minimal involvement. With an expected wait time of about 4 years, there's little incentive for new participants.
The backlog of entries can be reduced and prevented from building up if down votes are allowed and the most down voted entries are removed periodically.
This does not solve the problem, but does begin to address it while a "permanent" solution is sought.