lots_of_jpegs.py

import io
import picamera

class MyOutput(object):
    def __init__(self):
        self.file_num = 0
        self.output = None

    def write(self, buf):
        if buf.startswith(b'\xff\xd8'):
            if self.output:
                self.output.close()
            self.file_num += 1
            self.output = io.open('%d.jpg' % self.file_num, 'wb')
        self.output.write(buf)


with picamera.PiCamera() as camera:
    camera.resolution = (1280, 720)
    camera.start_recording(MyOutput(), format='mjpeg')
    camera.wait_recording(10)
    camera.stop_recording()