Assuming you're using Photoshop:
= New image 2x2 pixels
- Use the pencil tool to make a black/white checkerboard (either fill upper right and lower left pixels with black or vice versa
- Select all, then do Edit > Define Pattern (name is something, mine was "microchecker")
= Open the normally visible image (usually color)
- If it's the background layer, double click and give it a name. This layer will go on top of your stack when we're done
- Set the layer opacity to 75% (can be tweaked depending on your image, but this usually shows up well without totally killing the picture)
= For the hidden image (the one visible when highlighting in IE)
- The hidden image usually works best if it's a sharp black/white image (like a stencil), which is what I used in my example
- Invert the image if necessary (basically whatever parts are set to white will show up black when the picture is highlighted, so in my Vader example, the pic was inverted so vader was white on a black background)
- Bring this image into your main
PS document and put it on the bottom of the layer stack
= Make a new layer between the hidden pic (bottom) and main pic (top)
- Either paintbucket or Edit > Fill > Use Pattern > Custom > "Microchecker" *or whatever you named your 2x2 pattern*
- Set this layer's blending mode to Exclusion
- Done!
To try it out without goofing with IE, duplicate the middle pattern layer, put it on the top of the layer stack (above the main pic) and set it's blending mode to Multiply (which replicates the IE highlight effect). You can use this to make adjustments to the image without having to jump between IE and
PS to check your work.
Example (click to view fullsize as thumbnails won't show this effect):
