Upon further checking, I take back my claim on /usr/share/doc/texlive-doc/latex/beautybook/inner_pics/titleimages/songeven.png. Though this file does contain the 4 letters “caNv”, it's not a chunk. So beautybook is fine.
The 40 PNG files in /usr/share/doc/texlive-doc/latex/webquiz/examples do contain caNv as a chunk as per `pngcheck -v`. So the only culprit is webquiz.
