Prompted by the suggestion of Phil Grundig I might be adding transcode as a capturer in the future.
He also provided examples:
Frame extraction:
$ transcode -o thumb -y jpg,null -i somevideo.mpg -c 0-1,299-300
Will take frames 1 and 300 as jpegs
Frame extraction with size reduction:
$ transcode -r 5 -o thumb -y jpg,null -i somevideo.mpg -c 0-1,299-300
-r reduces size, e.g. -r 5 reduces size one-fifth.