All of the envelopes are identical. Absolutely the only difference is which envelope goes into which ballot box. Taking a photo of N-1 ballots for non-Dr. Evi

All of the envelopes are identical. Absolutely the only difference is which envelope goes into which ballot box. Taking a photo of N-1 ballots for non-Dr. Evil next to one sealed envelope only discloses that the Dr. Evil ballot was put into an envelope first. It gives no information as to which box that envelope was placed into later, and therefore leaks no information about how the vote was cast.