The mirrored negative images
I posted were created using GIMP, insprired by goplayer. These images were created from a photo using these steps:

  1. open the photo in 2 GIMP windows,
  2. open layer dialog and set opacity to 50%,
  3. set layer->transparency->color to alpha,
  4. transform one photo using image->transform->flip,
  5. color->invert (possibly boost saturation or tweek hues),
  6. copy,
  7. in window of other image, again, set opacity+transparency,
  8. paste and reset opacity,
  9. save (for safety)
  10. layer->remove alpha channel, and save.

Hope this works. Please leave a comment if you have a better way or if there are mistakes.