New File

Create a new file,make it 1 by 2 pixels and with a transparent background.

Zoom

Zoom in all the way to 1600%

Pencil

Now select the pencil tool and select the bottom pixel

Save Pattern

Now go to Edit>Define Pattern,add the name you will remember it as then select ok.

Adding Scan Lines to image

Open the image you want to add the Scan Lines to,then make a new layer and Edit>Fill then where it says use,select Pattern and under Custom Pattern find your image then press ok.

After

You should drop the opacity and fill depending on your image.

Done

Now your done,It should look something like this if you did it right