In the years since I posted that, twitter has changed some of their image processing pipeline, so that precise technique no longer works if you want to upload new files (last time I checked - and I didn't investigate further or look for workarounds).

However, it's much easier to embed data into PNGs, and the best twitter-compatible implementation of this I'm aware of can be found here: https://github.com/CleasbyCode/pdvzip

Edit: also, the standard `unzip` utility has become a bit stricter about the validity of input files - in ways which would have been possible to work around, but of course not retroactively.