Why not use one of those backyard auto light fixtures? They are cheap n will do exaclty what you want.

You'll have to install a relay so when the light goes off in the morning it will power your trip circuit instead of going dead. On the other side of the relay you can now power using batteries instead of wall juice to activate the motor or solenoid.

To adjust the amount of light that will trigger, they do come with a sensativity adjustment. Otherwise you'll just have to positioning it for a darker part of the sky or ground so it won't turn off too early, by covering the sensor you can also limit the light hitting it to eactly when it shoudl take the picture.Total cost... you may already have everything you need right in your back yard.

That timer idea is excelent, didn' t see it till after I posted.