Board Thread:General Discussion/@comment-26242189-20160601064323/@comment-27907368-20160602004112

That photo only shows up after the encounter with Clam Girl in a no EXP neutral route. (it could show up in all iterations after the Clam Girl chat, but it would be an assumption.) Knowing that the picture will only appear in that timeline after that dialogue, Sans probably met those people in the future. Perhaps, after the protagonist leaves, there was time for him to fix his time machine and slip that photo into the pile.

Those people are people we haven't met before. Knowing that Gaster is from the past and therefore assuming his Entry 17 is from the past (or present of Undertale, ugh), he probably wasn't talking about them.

That's what I've got. ^^'