Fanart ?

[graykin]

REFERENCING THIS PHOTO, SEEN ON THE RIGHT: http://www.giantbomb.com/tom-redwood/94-16466/

I dunno, I always thought it was Red. Yeah, the possibility of it being Frisk is completely reasonable, and I won't argue the point, but the narration at the time when the photo is displayed talks about the "madman Red." Also, there was a dead dog in Red's closet, which could very well be the dog you see the alleged Red hanging out with in the photo.

Not to mention that, on the website, they say the man is "Tom Redwood," which admittedly doesn't mean TOO much but I definitely wouldn't despair Mafer. There's, at least, enough ambiguity to make your assumption plausible.