Getting a snapshot from a video:
ffmpeg -i video_in.mpeg -f singlejpeg -t 0.001 test.jpg

Converting image frames into a video, for example 30 fps:
ffmpeg -b 2000k -r 30 -i %08d.ppm filename.mpeg

Convert a video from avi to mpeg:
mencoder -oac lavc -ovc lavc -of mpeg -mpegopts format=DVD:tsaf -vf scale=720:480,harddup -srate 48000 -af lavcresample=48000 -lavcopts vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9800:vbitrate=5000:keyint=18:vstrict=0:acodec=ac3:abitrate=192:aspect=4/3 -ofps 30000/1001 -o output.mpg input.avi

More information can be found Here.