Photos with a name ending in _n.jpg I have an iMac running High Seirra. I have quite a few photos on my mac that have names consisting of something like this: 11008553_991594160872293_5946103255845449374 and ending in _n.jpg. These look like normal jpegs but i don't understand the naming. What are these files?  They originally came from my iPhone.
My research shows that images downloaded from Facebook always ends with “_n.jpg” !

So you must have downloaded them from Facebook to your phone. 

iCloud then synced them with your Mac, unless you transferred them your self.