I wondered the same thing after using the Relive app. I'm pretty sure it's running off of time. I'm fairly confident that I had the sense to turn off location services for my photos, so I was curious as to how the app knew where I took the pic. When I uploaded one from Instagram I did after the ride, it put it at the very end, which leads me to believe the time stamp told the app the picture was "taken" near that location.

Edit: Ok, I think we might be talking about two different things. Do you have Strava set up to just use all pictures you take in a ride? I have to manually edit the activity and add photos.