ffmpeg -i zt%04d.png -c:v libvpx output.webm

ffmpeg version N-78335-g9ee4c89 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.3.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enable-zlib
  libavutil      55. 17.100 / 55. 17.100
  libavcodec     57. 24.101 / 57. 24.101
  libavformat    57. 24.101 / 57. 24.101
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 29.100 /  6. 29.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
Input #0, image2, from 'zt%04d.png':
  Duration: 00:00:02.80, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: png, rgba(pc), 256x256 [SAR 2835:2835 DAR 1:1], 25 fps, 25 tbr, 25 tbn, 25 tbc
[libvpx @ 000001462280dd60] v1.5.0
Output #0, webm, to 'output.webm':
  Metadata:
    encoder         : Lavf57.24.101
    Stream #0:0: Video: vp8 (libvpx), yuva420p, 256x256 [SAR 1:1 DAR 1:1], q=-1--1, 200 kb/s, 25 fps, 1k tbn, 25 tbc
    Metadata:
      encoder         : Lavc57.24.101 libvpx
    Side data:
      unknown side data type 10 (24 bytes)
Stream mapping:
  Stream #0:0 -> #0:0 (png (native) -> vp8 (libvpx))
Press [q] to stop, [?] for help
frame=   70 fps=0.0 q=0.0 Lsize=     140kB time=00:00:02.80 bitrate= 408.2kbits/s speed= 2.9x
video:69kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 102.946152%