Although posting your ballot may be illegal, it's unenforceable as a matter of law.
There's no way to prove that an image is your ballot.
The poll workers should be enforcing this at the polls, catching the "criminal" in the act, and it should not be enforced anywhere else. I'm not saying they should be prosecuted, but they should at least be stopping them.
Obligatory car analogy: Speeding at 100mph is enforceable, but posting a picture of your speedometer at 100mph should not be enforceable.