Export frames from a video file to images with max quality

avconv -i [source.ext] -vsync 1 -qscale 1 -r 1 -an -y [img%03d.jpg]

qscale goes from 1 (excelent) to 31 (worst)

More useful commands
http://www.roxlu.com/2013/002/converting-video-audio-using-avconv