Make sure its in RGB Mode. (Go to Image » Mode »RGB Color)
First select an image. I'll be using this one:
Make sure its in RGB Mode. (Go to Image » Mode »RGB Color)
Go to Layer » Duplicate Layer. Name it whatever you want.
Next, go to Filter » Pixelate » Mosaic.
Make the cell size around 18-30. This time, I set mine to size 25.
Lastly, using your layers box change the mode to 'Overlay'. Here's your finished product:
Next time test to see how all the other different modes look!