That's not all there is to it. Did you try uploading the resulting file to twitter?

I did not. This is just a response to the parent comment not to the post itself.

(Although if they do something with this file and you still are able to do this trick that would seem even more .. interesting on their side than leaving images below certain size untouched)

The zip offset fields would be wrong, so your unzip must have some error correction. Use "zip -A zipfile" on Linux to fix them. As mentioned though, image upload sites would likely scrub out the extra data.