Now well add a posterize effect to the image.
Activate the Background layer and duplicate the layer (Layer> Duplicate Layer or Ctrl+J). Move the new layer to the top.

Change the blending mode of the new layer to Multiply.
Select the top layer and open the Threshold tool (Image> Adjustments> Threshold). In the Threshold tool, drag the input slider towards the left or right to adjust the effect. Or if the default value of 128 is good enough, simply click OK.
