I use Paint Shop Pro 8, but any paint program with layering capability should be sufficient for this project.
First, select and load a photo into your paint program.
Click on the 'Layers' section of the toolbar; select 'Duplicate'.
The duplicate is now the active layer. Click on the 'Image' section of the toolbar, and select 'Mirror'. This will invert the image, right to left.
Go to the 'Layers' menu, and select 'Properties'. A submenu will pop up in it's own window. Set the blend mode to 'Difference', then click 'OK'.
This is the result:
If you're satisfied with the picture, copy and save it. If you don't make a copy first, you'll be saving over the original photo. Enjoy!