Dot Pattern
Ok, Open up a new canvas, 1x1 with a Transparent background. Then get out your pencil tool (again make sure its black), and zoom in to about 1700% And draw a dot to fill up the whole square.
Then go to Edit>Define pattern and call it Dots. To apply the patter, open up for Image, create a new layer, Go to bucket tool, Switch to pattern and click the newly made dot, Then Click your image. Mess around with Blending options to get a good effect.