To convert (part of) a movie to a series of JPEG files, I use MPlayer (download from
http://www.mplayerhq.hu or
http://mplayerosx.sf.net). I use the following command to save 10 frames starting at 555 seconds from the beginning of t.avi:
mplayer -nosound -vo jpeg -frames 9 -ss 555 t.avi
# creates 00000001.jpg to 00000004.jpg