libpng was upgraded from 1.2.x to 1.4.x a long time ago, and all ports that use libpng should already have been upgraded to newer versions or revisions thus forcing them to rebuild. Specifically, libpng was updated to 1.4.x in January in r75159, and openjpeg was updated to 1.4 in April in r77941 thus rebuilding against the new libpng.
But apparently on your system your openjpeg port has not been rebuilt since libpng was updated. I don't know how that's possible, unless you just haven't been updating your ports. So first, please selfupdate and upgrade all outdated ports.
sudo port selfupdate
sudo port upgrade outdated
If the problem persists, force openjpeg to rebuild:
sudo port -n upgrade --force openjpeg
This is described in wiki:ProblemHotlist#libpng
