Use QTKit and PyObjC to get a JPEG for an image's poster frame.

posterImageFromMovie.py

#!/usr/bin/env python
#coding=utf-8
"""
posterImageFromMovie.py, 2010 by Sven-S. Porst <ssp-web@earthlingsoft.net>
Uses PyObjC to extract the poster frame from a movie file and save it as a JPEG next to the movie. An existing file moviename.jpeg will be overwritten.
"""

from QTKit import *
import sys
import os

if len(sys.argv) == 2:
	path = NSString.stringWithString_(os.path.realpath(sys.argv[1]))
	movie, error = QTMovie.movieWithFile_error_(path, None)
	
	if movie != None:
		image = movie.posterImage()
		imagerep = image.representations()[0]
		data = imagerep.representationUsingType_properties_(NSJPEGFileType, None)
		
		if data != None:
			destinationPath = path.stringByDeletingPathExtension().stringByAppendingString_(".jpeg")
			result = data.writeToFile_atomically_(destinationPath, True)
			if result == False:
				print "Could not write JPEG file."
		else:
			print "Could not retrieve poster frame image data."
	else:
		print "Could not open movie file."

	
else:
	print "Please specify exactly one path to a movie file."