The threat model for coercion-resistance is providing proof to someone after you have cast your vote.
The threat model for anonymity is that an observer - either a 3rd party or someone colluding with the voting authority - that does not have access to the voting client itself.
Voting-at-gunpoint coercion as threat model cannot really be defended against because it basically implies that the attacker has full control over the voter. Even some scheme that would allow vote retraction/recasting wouldn't help since the attacker could simply keep threatening the voter until the election is over.
Let's say you're in the voting booth and have taken the picture. You can then do at least one of the following:
- Exchange your ballot for a blank one and fill it out again
- Fill in the check box and vote for your real candidate
- Spoil your vote