With -r

$ ./ffmpeg.exe -i redcliff.webm -r 1 test%d.png
ffmpeg version N-41027-ge02e58f Copyright (c) 2000-2012 the FFmpeg developers
  built on May 25 2012 13:09:41 with gcc 4.6.3
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-runtime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass --enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      51. 54.100 / 51. 54.100
  libavcodec     54. 23.100 / 54. 23.100
  libavformat    54.  6.100 / 54.  6.100
  libavdevice    54.  0.100 / 54.  0.100
  libavfilter     2. 77.100 /  2. 77.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, matroska,webm, from 'redcliff.webm':
  Duration: 00:02:14.09, start: 0.000000, bitrate: 733 kb/s
    Stream #0:0(eng): Video: vp8, yuv420p, 450x256, SAR 1:1 DAR 225:128, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      title           : VP8 video stream
    Stream #0:1(eng): Audio: vorbis, 22050 Hz, stereo, s16 (default)
[buffer @ 00000000003465c0] w:450 h:256 pixfmt:yuv420p tb:1/1000 sar:1/1 sws_param:flags=2
[buffersink @ 0000000000346860] No opaque field provided
[format @ 00000000003469e0] auto-inserting filter 'auto-inserted scaler 0' between the filter 'src' and the filter 'format'
[scale @ 000000000032ba40] w:450 h:256 fmt:yuv420p sar:1/1 -> w:450 h:256 fmt:rgb24 sar:1/1 flags:0x4
Output #0, image2, to 'test%d.png':
  Metadata:
    encoder         : Lavf54.6.100
    Stream #0:0(eng): Video: png, rgb24, 450x256 [SAR 1:1 DAR 225:128], q=2-31, 200 kb/s, 90k tbn, 1 tbc (default)
    Metadata:
      title           : VP8 video stream
Stream mapping:
  Stream #0:0 -> #0:0 (vp8 -> png)
Press [q] to stop, [?] for help
frame=  136 fps= 71 q=0.0 Lsize=       0kB time=00:02:16.00 bitrate=   0.0kbits/s dup=0 drop=3856
video:12891kB audio:0kB global headers:0kB muxing overhead -100.000000%