transcode [...] -y ppm
transcode [...] -y im -F tiff -c 0-20
writes 20 video frames as TIFF image format pictures to the files frame.000000.tiff, frame.000001.tiff, ..., frame.000019.tiff. Other possible and useful image formats are jpg and png. Please see the ImageMagick documentation for supported image formats. It is possible to change the prefix with option -o.