In the next release of RIOT please include the latest version of OptiPNG (v0.7.6 as of now):
optipng.sourceforge.net
On their homepage they stated "you are strongly encouraged to upgrade to the latest version" with "strongly encouraged" in bold for whatever reason. You can see the full changelog here:
optipng.sourceforge.net/history.txt
(RIOT comes bundled with v0.6.4)