From what I gathered, they are little thumbnails based on the face recognition of the camera and other downloaded images.
I opened a few of my .face files and oddly, one was a thumbnail image of part of a picture that I took of a vehicle...the thumbnail was the
wheel of the vehicle.
Another .face file I opened was a thumbnail of again, the
wheel, of a car wallpaper I had downloaded off of Zedge.
Wierd, I know, right? Why? I dont know. Can you delete them? I don't see why not, I deleted one of the thumbnails and I went back to the picture of where it came from, and there was no difference.
Edit: found this too:
http://forum.xda-developers.com/showthread.php?t=1929380&page=2