Take a peek at . It does say it's lossless compression. I admit I can't find any concrete info that says "lossless all the time" vs. "lossless after the first compression", but it does seem to in fact be lossless from what I can find.
Even better perhaps is , which says "non-patented (you betcha!), completely lossless compression". This also gives a better list than I did for why PNG is better (IMHO anyway).
Like you said though, your going to support both, so I certainly don't want to start a file format war
...and so I said to Mr. Gates: "$640 billion should be enough for anyone!"
