I'm thinking that the backyard auto light fixtures I've seen are 120V; there will be no external power available where the photo will be taken. Also, I'm not at all sure what the specs are for the sensors -- how repeatable they are, or what it would take to modify them so that they switch at the light level I want.

As to the timer: the sun rises almost 3 hours earlier in June than it does in January. That schedule is pretty well known, but with weather variabilty affecting brightness, it's really not possible to know on any given day when the light level will first reach the point where the photo must be taken.