hendrikus@10:~/Bureaublad$ ls
img1.png  img2.png  img3.png  img4.png

hendrikus@10:~/Bureaublad$ ffmpeg -f image2 -r 1/20 -i img%d.png out.avi
FFmpeg version SVN-r0.5.1-4:0.5.1-1ubuntu1.3, Copyright (c) 2000-2009 Fabrice Bellard, et al.
  configuration: --extra-version=4:0.5.1-1ubuntu1.3 --prefix=/usr --enable-avfilter --enable-avfilter-lavf --enable-vdpau --enable-bzlib --enable-libgsm --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-pthreads --enable-zlib --disable-stripping --disable-vhook --enable-runtime-cpudetect --enable-gpl --enable-postproc --enable-swscale --enable-x11grab --enable-libdc1394 --enable-shared --disable-static
  libavutil     49.15. 0 / 49.15. 0
  libavcodec    52.20. 1 / 52.20. 1
  libavformat   52.31. 0 / 52.31. 0
  libavdevice   52. 1. 0 / 52. 1. 0
  libavfilter    0. 4. 0 /  0. 4. 0
  libswscale     0. 7. 1 /  0. 7. 1
  libpostproc   51. 2. 0 / 51. 2. 0
  built on Dec 21 2011 18:37:21, gcc: 4.4.3

Seems stream 0 codec frame rate differs from container frame rate: 0.05 (1/20) -> 0.25 (1/4)
Input #0, image2, from 'img%d.png':
  Duration: 00:01:20.00, start: 0.000000, bitrate: N/A
    Stream #0.0: Video: png, rgb24, 512x384, 0.25 tbr, 0.05 tbn, 0.05 tbc
Output #0, avi, to 'out.avi':
    Stream #0.0: Video: mpeg4, yuv420p, 512x384, q=2-31, 200 kb/s, 90k tbn, 0.25 tbc
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
frame=    4 fps=  0 q=2.0 Lsize=     183kB time=64.00 bitrate=  23.5kbits/s
video:177kB audio:0kB global headers:0kB muxing overhead 3.338416%
hendrikus@10:~/Bureaublad$