Make a new layer (white) 2X2 pixels

Zoom in all the way, select the pencil tool and make sure it's one pixel.

Make one little square in the top right and then go to Edit>Define Pattern

Open an image you wish to add the dots too. Then, make a new layer, go to Edit>Fill and select pattern and find the corresponding pattern you wish to use. The, select the mode to overlay and lower the opacity if it is too much. You can make many patterns-most patterns are made with a trasparent background but white or black can also be very neat.