The image that I have chosen is

Let's try to add the scan effect to it. This can be done in two ways

1) Create a new layer and manually draw each line over it. Well that's what newbies do.

2) Here we will use one of the feature of Photoshop called as Patterns. Patterns are pre defined image that whenused fills up the layer with it self. So we are now going to create the scan line effect using this feature.

Step 1:

Start a new document of size width=4 and height=8 pixels, make sure that you select the background contents as transparent. Now zoom (z) a bit and use the Rectangular marquee Tool to select haft of the canvas. Now fill it with black colour using the Paint Bucket Tool (g).

Step 2:

Now let us set it as a Pattern. Go to Edit>Define Pattern and give it a new name say scan line pattern and say OK.

Step 3:

Now open the image on which we want to add the scan line effect in our case it is the skull image. Now create a new layer and go to Edit>Fill select the scan line pattern and say ok.

Step 4:

This is what I got. If we see clealy the lines are pretty dark and the skull image is not visible clearly.

Step 4:

Now all we have to do is reduce the opacity of the scanline pattern layer and we will be done.

That's it we are done. This is the result that I got.