diff --git a/doc/Makefile.am b/doc/Makefile.am
index 2512b7142..f0b0adc16 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -108,3 +108,8 @@ EXTRA_DIST = \
   $(ORG_FILES) \
   $(srcdir)/org-stamp \
   $(srcdir)/userdoc
+
+dist-hook: optipng
+.PHONY: optipng
+optipng:
+	-(optipng -v >/dev/null) && optipng -o5 $(distdir)/userdoc/*.png