My understanding on photos is:
1. You can set a camera time whatever you want, but if these are cell phone pictures, you can not change it. So I think you mean a time embedded in a file in a cell phone.
2. An investigator would need an original or maybe a good copy of the photo to see if it was altered. I can not tell if a photo has been altered, but I gather it does not take great expertise.
3. That one accused can reportedly show that he was not at the party and that the other can show he was making calls and left early may matter more than the photos.