I saw someone who wanted to take my picture, so I went up to interact with them. Unfortunately, they were looking the other way and I accidentally grabbed them and then broke their neck (I meant to toss them, but pushed the wrong key, I swear!). Am I out of luck, will I be able to get that photo op later or did I permanently miss out on that opportunity?
Yes, just return later and they'll be good as new. There's no way to permanently miss these ops.
I'm not sure what exactly constitutes "later," as leaving the immediate area and returning sometimes works and sometimes doesn't. For certain, saving and then loading your save will reset them.