Step 1.) Make a new image size 3x3 and make the background transparent.
Step 2.) Take your zoom tool and zoom in on the image as much as you can.
Step 3.) Now make a line going diagonal on the canvas like this.

Step 4.) On the main tool bar at the top of the program click
Edit>Define Pattern. You should get a pop-up like this.

Step 5.) Open up any image. Now go to
Edit>Fill When the pop-up comes up use these settings.


Your finished. Now you can add scanlines to and image easy as that.