extract iframe from mpeg file

for really small thumbnail 
ffmpeg -y -i x.avi -vframes 1 -ss 00:00:10 -an -s 120x90 -qmin 20 x%d.jpg

for high quality output
ffmpeg -y -i x.avi -vframes 1 -ss 00:00:10 -an -vcodec png -f rawvideo -s 320x240 x.png