To see the hidden image tilt the computer screen, look at the screen from a side angle, step away from the screen, or shake your head really fast. The image is almost completely invisible if viewing strait on and semi close.

I was looking at optical illusions when I came across "John Lennon optical illusion." See link below for the illusion.
http://www.moillusions.com/wp-content/uploads/1.bp.blogspot.com/_cxmptAPYR-s/Rgaa5KgQbWI/AAAAAAAAAiM/5daHyzsad5s/s400/Lennon+Illusion.jpg
I looked on the internet for a tutorial but couldn't find one. After playing around with Photoshop I came up with this Instructable.

Here is the finished file.

This can be done with a photo or text.

Tools needed:
-Adobe Photoshop
- A Photo

Step 1: Black and White Lines

Open a new Photoshop file.
Make the size 4 px wide and 9 px long (image 1.)
Zoom in if needed to see.
Use the Single Column Marquee tool to select 1 column of px (image 2.)
Use the paint bucket tool to fill column. Follow the color scheme, black gray(9f9f9f) white gray(9f9f9f), as seen in image 3.
Define the pattern Edit / Define pattern (images 4 and 5.)

Step 2: The Hidden Photo

Select a photo or text and open it in a new Photoshop tab (This works better if the photo is a close up shot.)
Use the quick selection tool to select the background of the photo and press delete to remove the background (image 2.)
Image/ adjustments/ Desatureate to remove the color (Image 3.)

Step 3: Finishing Up

Create a new layer Layer / New / Layer / Ok (image 1.)
With they new layer selected.
Fill the new layer with the pattern Edit / Fill (image 2.)
Find the pattern you created earlier click ok (image 3.)
Your screen should look similar to image 4.
Drag layer one on top of layer two (image 5.)
Make sure the foreground color is gray (#9f9f9f) and the background color is white (image 6.)
Go to Filter / sketch / Photocopy (image 7.)
The settings should be similar to image 8.
Change the blending mode to multiply.
Change the opacity to around 50% or till desired effect is reached.

If some of the images still shows through you can use a curves adjustment to lighten the darker areas.

Step 4: Final Hidden Image

To see the image you can tilt the computer screen, look at the screen from a side angle, step away from the screen, or shake your head really fast.

Thanks and If you have any questions post a comment or send me a message.