The VideoLAN Forums

Image video output
--image-out-format={png,jpeg}
Image format
Format of the output images (png or jpg).
--image-out-width=<integer>
Image width
You can enforce the image width. By default (-1) VLC will adapt to
the video characteristics.
--image-out-height=<integer>
Image height
You can enforce the image height. By default (-1) VLC will adapt to
the video characteristics.
--image-out-ratio=<integer>
Recording ratio
Ratio of images to record. 3 means that one image out of three is
recorded.
--image-out-prefix=<string>
Filename prefix
Prefix of the output images filenames. Output filenames will have the
"prefixNUMBER.format" form.
--image-out-replace, --no-image-out-replace
Always write to the same file (default
disabled)
Always write to the same file instead of creating one file per image.
In this case, the number is not appended to the filename. (default
disabled)