How to extract a thumbnail from a video file with mplayer
mplayer 123.avi -ss 1 -nosound -vo jpeg:outdir=. -frames 2
We use the -ss switch to jump to the first second of the video; specify the time in either seconds or hh:mm:ss. We tell mplayer that we want two frames with the -frames switch.
Make sure jpeg is in the list of supported video output formats:
mplayer -vo help