1. Open photoshop
2. File > New > Width - 3px & Height - 3px
3. Create a new layer
4. Delete the background layer (should now have a transparent background)
5. Right click on your paint brush tool in your tool box and select Pencil
6. Zoom in on your canvas 3200%
7. Fill in the upper left corner of the blank background with 4 strokes of your pencil (whatever color you prefer - black in this case, I'm assuming)
8. Edit > Define Pattern > Name your pattern > Hit OK
9. Open your photo
10. Create a new layer
11. Edit > Fill > Use: Pattern > Select your newly created pattern
Depending on how large your image is, you should get something like this (note: the larger your image is, the larger your square will have to be, so it's best to learn these steps from scratch anyway):
File > New > input pixel size of the image you want to do this effect on > OK
Filters > Render > Pattern > Grid
Your width & spacing will depend totally on the size of the image!
Colors > Invert
Layers > Transparency > Color to Alpha
If the color bar between "From" and "to alpha" is not white, click on the bar and set the V slider all the way to the right. Click OK
One more OK to make the grid
File > Open as Layers > load image you want effect on.
Layer > Stack > Reverse Layer Order
File > Save As... > create a new filename > choose image format (usually jpeg) > Export > Save.