Here is the code I used

http://pastebin.com/jA1tmMuE

EDIT: It's C#

Here you go, this is the JavaScript way of getting the data out of the alpha channel. I commented the code extensively just for you and you can look it up via your browser.

www.alaric.us/misc/puzzle.html

If you don't want to open an untrusted site or don't know how to view source in the browser here is the same thing in pastebin. The only drawback is that you won't be able to see it work before your eyes. =)

pastebin.com/P8iQ6WBW

Enjoy!

Attached is the C++-code I used (to actually view it download the file, rename to <something>.7z and uncompress it (GOG doesn't allow files with non-image extensions)). There may be some configuration necessary (relevant xkcd) to run it (mainly selecting the compiler).

I know how to view source, thanks for the comments. :)

Thanks to you all for providing your code!
At some point I came up with the actual visible alpha channel data too, but couldn't extract anything useful from it by eye. It's nice that you had different approaches, the finished examples are less complicated than I imagined. :D