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One can still vote on answers to closed featured image questions.
Since these threads are intended to capture a selection process, once that process ends, we should stop further voting so the votes accurately show what the tally was.
I don't think this is really an issue. The submissions receive upvotes during the submission period, because others like the photos. It's not like people don't stop looking at the older ones, so they should still be able to upvote.