Zymic

Open up your image you want to apply the scanlines to, I used the one below:

Now make a new document with a width of 1 pixels and a height of 2 pixels (with a transparent background). Now zoom in using the Zoom tool around 1600%. Once zoomed in grab the pencil tool and draw 1 black pixel dot on the top half of your document like below:

We now need to define the pattern, go to [ Edit > Define Pattern ], name it "Scanlines" and press OK. Now bring back your original image you want to apply the scanlines to and make a new layer [ Layer > New > Layer ]. Grab the Paint Bucket tool. Now look at the top of your screen and select "Pattern" from the fill drop down box and select the pattern you named "Scanlines" from the pattern drop down box like below:

Now fill your image in and it should have scanlines applied to it! Simple play around with the layers opacity % or it's blending modes to get a better more smooth result.

Now fill your image in and it should have scanlines applied to it! Play around with various blending modes and opacity levels to get better results!