You can transform an image shot in the daytime to one that appears to be shot in the night time, in Photoshop Elements or CS. All you will need is a picture of a subject, preferably strongly lit by the sun on one side, with a clear sky. A picture of a moon will also add to the authenticity.
Begin by opening the start image, and bring up the layers palette (Window > Layers). Click the Create New Adjustment Layer Icon and choose Hue/Saturation. Bring the saturation slider down, until the image tones are muted, and almost greyscale. Next, use the Hue/Saturation drop-down menu to select the blues. Bring the saturation down to cool the image and add a night tone, and use the lightness slider to tone the sky down.
Go to the Create New Adjustment Layer Icon and choose levels. Drag the white slider left to create a harsher light, and drag the midtones slider to the right, to darken the midtones and shadows, and create a more harshly focused moon-like light.
Next, you need an image of the moon! Open your moon image, and Choose Select > All (or Ctrl/Cmd + A) > Edit > Copy then go to your day/night image, and paste the image using Ctrl/Cmd + V or Edit > Paste. The image will be pasted in the middle. Ensure the moon layer is on the top of the layer stack, and drag and position the moon as necessary to correspond with the light in the image. Then, change the moon’s blending layer to Screen. This avoids the hassle of trying to isolate the moon from it’s original background. If the moon image looks too large to be natural, choose Image > Transform > Free Transform, and you can rotate or resize the moon as necessary. Do ensure, however, that you tick the constrain proportions box in the options bar, otherwise you will find that the moon may be stretched and distorted!!
If necessary, go to Layer > New Adjustment Layer > Photo Filter, and select a Cooling Filter. You can then cool the image to add more of a cool, nocturnal tone to the image.