There is a new bug in libpng right now. When you want to see which package is using libpng (or better: which package depends on libpng), you can do a reverse dependency search:
apt-cache rdepends libpng12-0
This shows you a list of packages which directly depend on libpng. It lists all the tools which use the shared library of libpng.
You won’t be able to find out static compilations of programs using libpng, tho.
Works in Debian and Ubuntu.
(Picture by redbubble.com.)