Ok, so who did it?

Except not at all. It's not ambiguous nor is there any reason for the junk man to kill and lie about the girl. It's clear the son killed her, although he wasn't quite aware of what he was actually doing.

I just watched this film and enjoyed it. However, after the Mother going to see JP in jail, I couldn't decide whether it was the junk collector or the son - although we couldn't 100% say with certainty that JP didn't do it, for the sake of discussing the other two suspects, I'm going to discount him.

Although the last few scenes suggest that the son was responsible, the one piece of evidence I can't really get my head round is why was there a photo of the junk collector on the girl's phone?

It's true that the girl and junk collector may have had a liaison previously and she may have been on her way there to meet him, although it doesn't look that way to me in the film based on how she kind of side steps - as if she'd intended to keep walking. If the junk collector had indeed recalled his story with 100% truth, when he had seen the girl he knew (as the photo must have been taken before she died as there is no reason he'd implicate himself after by taking his own photo and not keeping the phone!), why did he not intervene? Even if he chose not to intervene, why did not go after the son to ensure his capture, or call the police earlier to give his testimony (he waits until the mother is round before jumping up, saying he should call the police)? I wondered if he knew the mother was onto him so concocted some of the story and then showed like he was trying to prove his innocence by saying he'd call the police.

I agree that the son most probably was seen by the junk collector at the scene of the crime as he knew the way he massaged his temples, and the audience knows the son was there BUT the fact that he saw the son massage his temples doesn't make the rest of his tale true. All we know is that he has seen the son do this at some point before (potentially the enactment or he's seen him around town).

A possibility is that the son did throw the rock in retaliation, massaged his temples as he was frightened and shaken up, but then left the scene. The girl may have entered the house, or the junk collector gone outside to see if she was ok - this girl does not have a good family - he may have played a gentle father figure (previously she said "I hate men" and judging by other comments, it seems she's mainly slept with high school guys so may have seen him differently). She may have then taken a photo of him and for whatever reason, he kills her (possibly following rape - we don't really know much about his character, other than that he's a bit of a loner). These are of course just speculations but they are possibilities as I find it highly significant that guy's photo is on her phone which is never explained.

Going back to the last scene again where the son is commenting about why someone would hang the girl up for all to see, again this could still potentially be something the junk collector might have done due to what we do know about his character - he seems to be a nobody (the mother barely acknowledges him when she purchases an umbrella) and lives outside the town, yet putting the body on show is something the whole town gets to see.

I think the film makers may have intended for it to be the son as it makes a better story, although that's subjective in itself, but I don't believe there is anyway from concretely knowing either way as to what really happened. I find it interesting from a psychological perspective that which we assume to be true.