First, you should create, buy or get a picture which you want to illuminate. I bought some vector-images from Fotolia and arranged them to get my template.

I chose six of the bigger stars to be illuminated and created a drilling template for the wooden frame.

Buy some wood plates and drill the holes. Then add the frame. 

Because of the easy assembly and translucency, a printed cloth is the best looking solution for attaching the picture on the frame.

Connect all LEDs (WS2812) and the light sensor with wires.

Use the Arduino Nano (or a Clone), the RTC and a power supply to control the LEDs.

I added some sensors and potentiometers to parameterize the nightlight. Because of the LDR (mounted in the frame), the circuit can adapt the brightness. One button helps in setting the moment of day-/night-switch ('ZEIT') and the RTC module keeps the time. Six potentiometers parameterize the brightness of the LEDs (during darkness and during daytime), the threshhold value (light/dark), the brightness of the stars and the speed of the sparkling.

To be safe: please test if your RTC is built for the desired battery type. My module from china has a CR2032 battery but also has a battery loading circuit (not good). So I desoldered this part (a diode) and also the red power-LED. More infos here or here (german).