As far as I know, the oldest photo of any of my direct relatives is this one, of Lydia Richardson (l/k/a Lydia Cox after her marriage to William Cox).
She was my 3rd Great Grandmother -- her lineage is the first part of the line from this post. She was born in 1832 and looks to be maybe mid twenties here. There is no date on this photo so I'd just have to guess its somewhere between 1850 to 1870. This is as far back as I've gotten to any direct relative of mine.
[UPDATE - this post had listed a photo of one of my wife's ancestors, Rebecca Mendes Machado Philips. After further research, I am not able to verify that the photo listed was, in fact, a photo of her. I've removed that portion of the post.]