An interesting question, and an expert answer from a friend of mine. If you are interested in elections and voting and how they can and cannot be bought, you might find this intriguing. By the way, this scheme would never work in the US as we have many mechanisms that would prevent that, but these might apply in other countries that are not as sophisticated.
Say a bad guy made an offer secretly to the population that he would give each voter $100 for a vote cast for himself. Say they are using optical scan ballots. Say that cell phone cameras are easily snuck into the polling booth.
If you were the bad guy, what proof would you ask for so that people couldn't trick you and collect lots of $100 for votes that they really didn't cast. And if you were the government or an activist, what would you tell people about how to trick the bad guy and collect a bunch of $100 bills?:
Here's the analysis:
Before the Australian ballot, it was easy! Anybody can print a ballot, lots of straight party ballots printed. You go to your party boss in your polling place, he gives you a ballot, you put in the box, simple. When the party bosses were excluded, well, you had to get your ballot from him, and minions could observe you not getting a different. With some slight of hand you could trick them, but still very effective b/c most people won't attempt the slight of hand under threat of kneecapping. The almost as soon as Australian ballot was adopted (you might get a blank ballot from a government official in the polling place and mark it in the polling place), chain voting was invented.
Now, today, chain voting is too pesky and low throughput, how about we use the voter's digital camera in the polling booth! The can take a picture of the ballot that they marked as instructed by the boss. You show the boss a picture on your camera, he gives you money and/or spares your kneecaps. But wait! The digital photo can be faked? Hmmm.
I think that is the stage you've set. There are two main questions. What methods can the boss put in place to increase the difficulty of faked photos? What measures can election officials take to make it more difficult for real photos to be produced?
A separate question, new to me: if you were the government or an activist, what would you tell people about how to trick the bad guy and collect a bunch of $100 bills? You bear in mind that it is not just money. The deal might be this: you show me you voted right and I'll give you money; you don't show me, and my goons bust your kneecaps.
Well, if I were the gov't, I would be forbidding the use of cameras or cellphones or any kind of recording device in polling places, rather than telling people it is OK. Allowing recording devices in the voting booth is creating the opportunity for vote intimidation. You never want that.
So let me go back to the two main questions. I would suggest to election officials that ballot marking be done in three sided carrels made of translucent plastic that will mask a view of the ballot being marked, but allow a view of the use of recording devices.
There would need to be lots of training both of voters -- we really don't want you to using recording devices! -- and poll workers to intervene by asking a voter to please mark a ballot again, because this one you marked and then did some weird stuff in the carrel that looked like taking a photo.
The boss has a harder time. Clearly a photo of a properly marked ballot won't do, b/c anybody can make one of those. The photo would need to include something that showed the ballot and me, together. So probably it should include my hand as something that should be unique and distinguishing. Maybe the boss could stamp my wrist with a unique number, in ink that takes days to wear off. Even so, I could prepare a photo combining a properly marked ballot, and my hand, but not the ballot that I cast! If pre-printed ballots are freely available, then I can prepare a photo that the boss expects, but still vote a real secret ballot in the polling place. I could even make a fake picture in the voting booth, using a pre-made ballot, but then marking a real blank ballot and casting that. From there you are in a spy vs spy sort of games with more boss requirements on the photo, making it more difficult to fake, but also more difficult get away with in the real voting booth.