http://bugs.gentoo.org/305101
http://repos.archlinux.org/wsvn/packages/gd/trunk/libpng14.patch

patch in upstream repo already

--- gd_png.c
+++ gd_png.c
@@ -149,7 +149,7 @@
 	return NULL;
   }
 
-	if (!png_check_sig (sig, 8)) { /* bad signature */
+	if (png_sig_cmp (sig, 0, 8)) { /* bad signature */
     	return NULL;		/* bad signature */
 	}