I wish we knew exactly what the police said back to him after he said that if that pic was on the girls' phone, then it wasn't him. Maybe we do know, IDK. But seems to me they should've said something back to him about this being a pic of him in the background, not necessarily a pic they took of him specifically on purpose. Why would he need to vehemently deny that he was in this pic, even if it came from their phone? He shouldn't have immediately thought this was incriminating.
You can see he has his head down in the pic, concentrating (or pretending to concentrate) on the rickety planks of the bridge he was walking on. So he wouldn't necessarily have seen 2 girls ahead of him taking selfies etc. (or it was something he could've plausibly denied.)
Nothing wrong with that being a pic of him, even if it came from their phone, if he was just caught in the background of their pic. (I realize they were actually trying to surreptitiously video this creepy guy they thought might be following them, but that didn't necessarily have to be the case, and the cops could've sold it to him as it just being a shot of him they happened to catch in the background of one of the photos they took of themselves on the bridge that day.)
Him denying that it could be him in the pic if it came off their phone, when there's innocent explanation of that, looks guilty to me.
He also kept saying that it couldn't be him because he never met them! ("I never met those girls.") But why didn't the cops say it didn't matter if he ever met them or not? No one's saying he did. He wouldn't have to have met them for them to have found this photo of him on their phone. It doesn't look like a photo of someone they had ever met. If they'd met him and had a photo of him, it would probably be a posed photo of him smiling at the camera, maybe with one or both of the girls in the pic next to him, like you would do if you met some guy and wanted to take a pic of him shortly after. There's no reason for him to say he never met these girls. No one said he did, as in them ever having been introduced to each other or just stopping to make small talk as they passed each other while walking or anything like that. If he had met them and they had a pic of him on their phone, it would be an entirely different kind of pic, not one that looks like he's just someone they caught in the background of a pic they took of themselves.
That should look like just an innocent pic of him in the background which he should not have any reason to deny as he did. He didn't at the time know the police had more to this pic, which was just a still shot of the video in which police say his voice is heard ordering them to go down the hill, where they were later found killed. He didn't yet know anything about all that, but he still had to deny there was any way that could be him if it came off their phone. Incriminating.
I feel like the cops should've pressed him on this aspect.