Just tried making my own.
For photoshoppers, here are some steps to do this:
Put your primary image on layer one.
Put a checkerboard pattern of alternating black and null pixels on layer two. The top left pixel should be null, the one to its right and below it should be black.
Put your ghost image on layer three. You might want to lower the contrast a bit, or pick an image that doesn't have a lot of high-contrast edges in it, so it isn't as apparent in the final image. (The one I chose is rather apparent...not an ideal candidate).
Put a second checkerboard pattern of alternating black and null pixels on layer four, but shift it one pixel to the right of the prior checkerboard pattern. So the top left pixel on this one is black.
Now change the layer style from Normal to Lighten on layers two and four.
Save as a GIF, and you should be set!