If you’re using Internet Explorer for Windows, try loading this image and typing Ctrl-A to highlight it. Is it magic? Or EEE-VIL?! (An explanation of the trick follows.)
How to create your own super-secret images with Photoshop… Create a 2×2 pixel checkerboard pattern with the pencil tool, select the four pixels and select “Define Pattern” from the Edit menu. Now select your foreground layer and select the whole image. Hit “Q” to go into Quick Mask mode, and select “Fill” from the Edit menu. Select “Pattern” from the Use dropdown, set Blending to 100% and click OK to fill the mask with our checkerboard. Hit “Q” to go back to Standard Mode and hit Delete to remove every other pixel from the foreground image. Now adjust the opacity of the background layer until it fades from sight. Save your image as a high-quality JPEG, and you’re done.
(Thanks for the trick, bakabon38!)