An autostereogram is a 2d image that when looked at properly, shows a 3d image.
This instructable will show the many steps to completion.

Step 1: Download and Install a Stereogram Maker.

I used STMaker, a freeware stereogram creation program. It can be downloaded at:
http://www.swiftgear.com/stmaker/features.html
(I am not the creator, just a user)

I installed it and opened it up.

Step 2: Choose Your Image

I used photoshop 7 for this, as well as an Instructables Robot silhouette.
The next few steps will be preparing the image to create a nice, rounded end image.

Step 3: Editing the Image

To make it seem to have depth, we will use a gradient effect, blending the black into the white.
This will make the stereogram maker give it depth when it processes it.
I then used the select tool to select the area outside of layer 1, popped back to layer 2 and filled the selection with black.

Step 4: Adding Blur to "round It Out".

I then used a gaussian blur of 3 pixels, to create this image. Go ahead and flatten your image and save it. I chose robot.jpg.

Step 5: Back to STmaker, Open the New Image.

Click on the icon to open an image. Select your image and a small window will pop up. Click on "as range image" and click "OK".

Step 6: Making a Pattern

Click on "Create random coding pattern" on the tool bar, it'll give you two options. I chose color, but greyscale works just as nice.

Step 7: Time to Make This Bad Boy!

Now that everything is set, click on "create stereogram", it'll take a few, but not too long if your computer is nice.

Step 8: View Your Artwork and Save.

It'll pop up your piping hot stereogram for you to see. At this point, right click it and click on save, that way you can share it with friends.

In retrospect, I should have shaded the arms better, so they stood out.

Enjoy.