I don't know from where you got this libpng 2:1.6.9-1 rpm. It has the epoch field as "2". Hence it has higher precedence than version number so it won't allow the user to upgrade. You need to download the Toolbox libpng manually and forcefully upgrade it.
rpm -Uvh --force libpng-1.6.27-3.aix6.1.ppc.rpm