Old photos popping up as new

I am unable to delete pictures on my galaxy note 5. Recently and ex of mine from 3 years ago messaged me on Facebook and all her pictures she sent back then popped up as new in my gallery. My girlfriend broke up with me because she thinks they are recent and I was cheating on her. Is there anyway I can prove their old, she doesn’t believe they are old pictures she thinks I just recently saved them on my phone because they popped up first in my gallery since my ex decided to message me. :/