It’s fairly easy to blend two images in GIMP. All you have to do is: Open the two Images in GIMP. Blend the images using layer opacity/blend mode/layer mask. Export the resultant image as JPEG/PNG.
Right. What I have is two pictures taken from slightly different positions. I needed to alternate pixel rows between each picture to test the passive 3D on my new monitor. (It seems to rather suck, ...