1) Get original video
http://download.blender.org/peach/bigbuckbunny_movies/big_buck_bunny_480p_surround-fix.avi

2) Generate videos of X seconds with something like this
ffmpeg -i big_buck_bunny_480p_surround-fix.avi -ss 00:01:00 -t 00:00:5 -an bigbuck5s.mp4
ffmpeg -i big_buck_bunny_480p_surround-fix.avi -ss 00:01:00 -t 00:00:20 -an bigbuck20s.webm

3) Then convert the video to a gif image with the script in this directory
./video2gif.sh bigbuck5s.mp4 bigbuck5s.gif

4) And finally convert the gif to an apng with:
# sudo apt-get install gif2apng
gif2apng file.gif file.png