diff --git a/HACKING b/HACKING
index 1ba7f1c9e..df1dae365 100644
--- a/HACKING
+++ b/HACKING
@@ -46,8 +46,6 @@ only for certain operations (like releases):
 
   pandoc    used during Debian packaging for the conversion of
             IPython notebooks to html
-  optipng   used during "make dist" if present, to optimize
-            distributed png images
   R         used by some example in the documentation (the
             documentation will still compile without R, but that
             example will appear broken)
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 2d1053be4..1f523c962 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -147,10 +147,4 @@ EXTRA_DIST = \
   $(srcdir)/org-stamp \
   $(srcdir)/userdoc
 
-
-dist-hook: optipng
-.PHONY: optipng
-optipng:
-	-(optipng -v >/dev/null) && optipng -o5 $(distdir)/userdoc/*.png
-
 DISTCLEANFILES = org/plantuml.jar