libpng-1.6.4 is available from
ftp://ftp.simplesystems.org/pub/png/src/libpng16
and from
http://libpng.sf.net/index.html

Changes since the last public release (1.6.3):
  Added information about png_set_options() to the manual.
  Delay calling png_init_filter_functions() until a row with nonzero filter
    is found.
  Fixed inconsistent conditional compilation of png_chunk_unknown_handling()
    prototype, definition, and usage.  Made it depend on
    PNG_HANDLE_AS_UNKNOWN_SUPPORTED everywhere.

Glenn