So, I know that this question has already been answered before, however, the steps to actually create the image (the black cat/white cat image) were pretty unclear and it might also be somewhat outdated by now. Apparently you need to adjust the color levels of whatever two images you want to use to "satisfy the relative lightness condition a ≤ b" but it doesn't actually say how to do that. So if anyone could explain step-by-step how to create a PNG that looks like either the first or the second image I've provided down below depending on whether the background color is black or white using either GIMP or Photoshop, I'd be very happy. (Keep in mind that I'm pretty much a beginner at both GIMP and Photoshop, so please explain in detail what to do, what buttons to click, etc.)
What it means is that you need to do a levels adjustment on the images so that one is bright, and the other dark. There's no specific setting. You can just eyeball it. This is so that you won't see ghosting of the two images together. The starker the brightness difference, the better the result.
So for example, here are your two images with some levels adjustments
And here is the finished result showing the final edit over a white and black background.