We try to combat this in two ways in Germany (not that it currently poses a problem but still): 1. Voters can destroy their ballot before putting it into the box and get a new one for any reason thus making it possible to create fake proof. 2. Taking pictures inside the booth is explicitly prohibited (which stated on signs in the booth) and if the poll workers notice you taking a picture they require you to destroy the ballot and fill out a new one. I actually had to do this to a women last election who wanted to post a picture on social media – and didn't turn off the shutter sound.
This requires the poll workers to be somewhat impartial. If they are not all bets are off. In the German Democratic Republic voters where socially pressured (by official policy) to not use the booths at all.
Taking pictures in the booth isn't actually prohibited. You can do it but the helpers will probably take your ballot and destroy it, then give you another one.
(An exception is when you are a government delegate voting for chancellor, in which case you pay a fine of 1000€, especially after the president has clarified this rule to new delegates repeatedly)
We've actually had members of parliament in the Netherlands proudly show of the selfies they took in the ballot booth with their ballot on Twitter, encouraging people to take selfies (nicknamed 'stemfies'; a portmanteau of vote ('stem') and selfie) to show that (and how!) you voted on social media.
The election council and a lot of privacy-conscious citizens weren't amused to say the least, but a subsequent ruling by a judge turned out that it isn't strictly speaking illegal, because the law doesn't explicitly prohibit photography in the ballot booth. So until parliament enacts a law that forbids it (like Germany), we are stuck with this rather dubious phenomenon.
At least we've banned voting computers for a good while — the ones we had ran closed source software and the votes cast couldn't be verified by the voters or the election council — so there's that.
It depends on your definition (there is no fine or anything) but it's now explicitly prohibited and poll workers are required to reject such voters (and give them a new ballot), see § 56 Abs. 2, 6 Nr. 5a BWO.
This requires the poll workers to be somewhat impartial. If they are not all bets are off. In the German Democratic Republic voters where socially pressured (by official policy) to not use the booths at all.