Photograph with Pixel Overlay

1) Duplicate your layer. The shortcut for this is 'Ctrl + J'.

2) Make sure the duplicated layer is selected.
Go to Filter - Pixelate - Mosaic.

3) Choose the cell size. I am choosing '20 square'.

4) On your pixelated layer, set the blend mode to overlay and opacity (transparency) to 70%.

 5) You are finished.