Hey @dxhtml! Thanks for your reply. Like I wrote in the blog post, the answer is to take the filenames (excluding the file extension) of the first two images and xor them. You can either do it by manually converting both to binary, xor them, and convert the result back. Alternatively, you can use the website suggested by @tarekskr: http://xor.pw/
I hope you found this information helpful, if you have any further questions, just let me know.