Make a 1 pixel by 3 pixel transparent image.

Take the pencil toolthen press x then d on your keyboard to reset the colors. Now make a 1 pixel box like the image below.

Go to Edit->Define pattern name the pattern Scan lines.

Now choose the paintbucket and click on "foreground" and change it to "pattern". Then for the pattern select the 1x3 image you just made previously and play around with the opacity for your preference. Then go back to your image you want the scanlines on and use the bucket tool to apply it.