A DPDT relay would allow you to reverse the motor but it wouldn't allow you to shut if off without including more components to control that.
By using two separate relays you can use limit switches to shut off the motor when the end of travel is reached and simultaneously reset it so it is ready to be turned on in the other direction. Using a pair of relays with the common terminal connected to the motor leads, the NC terminal connected to ground and the NO terminal connected to B+ is the way that a number of automobile manufactures wire their door lock motors.
Unfortunately that would also mean that you would need two photo resistors, one to activate the "forward" or "up" relay and one to activate the "reverse" or "down" relay.
Here is a little information on how to use a photo resistor in a voltage divider circuit to cause the output voltage to either increase as the light increases or increase as the light decreases.
http://www.acroname.com/howto/photor...oresistor.html
You would need to select the proper resistors to create a "dead band" to prevent the system from trying to activate the "forward" and "reverse" relays at the same time or to fluctuate between the two states.
Page 3 of this document shows a circuit using a photo resistor in a voltage divider circuit to drive a transistor which switches a LED on and off based on the amount of light.
http://www.engr.sjsu.edu/bjfurman/co...stor-atmel.pdf Replace the LED with the relay coil and the resistor with the limit switch wired to the NC and common terminals and you have the circuit that would activate the motor in a desired direction when the light is bright enough and then shut off the motor when the limit switch is activated.
By swapping the position of the photoresistor and standard resistor in the voltage divider circuit it would activate the relay when the light dims enough.
You need to select a transistor that can handle the current and voltage needed to activate the coil of the chosen relay.