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This reminds me of how some government redacted .doc files by using a virtual "black marker", which of course leaves the text in tact.
Like in BALCO case? http://www.sfgate.com/c/acrobat/2006/06/22/BALCO_quash_subpo... (page 6, just highlight the text)

Or AT&T? http://news.cnet.com/2100-1028_3-6077353.html

There was also one more subtle source of a leak. some versions of MS Word will try to preserve/serialise the edit history after you save the document. Google filetype:doc and a simple tool were enough: http://lcamtuf.coredump.cx/strikeout/

It's a neat idea, but wouldn't work for any image format that's actually used. The code targets PPM, which is an ASCII-based image format. In a binary format like JPG or PNG, different values of zero would be indistinguishable.
This is the third or fourth time this has appeared on HN this year...
do you have links to the other ones - would be useful to see the other comments.
The only thing I found was http://news.ycombinator.com/item?id=1208097