This BASH script uses FFMPEG to create thumbnails from a video. The thumbnails are then stitched together as a gallery using ImageMagick.

com.vsubhash.bash.create_thumbnail_gallery_for_video.txt

#!/bin/bash
# BASH script to create a 3x3 thumbnail gallery for a video
# Accepts the pathname of the video as argument ($1)

###########################################################
# Floating point number functions by Mitch Frazier
# Adapted from 
# https://www.linuxjournal.com/content/floating-point-math-bash
###########################################################

# Default scale used by float functions.
float_scale=2

# Evaluate a floating point number expression.
function float_eval() {
  local stat=0
  local result=0.0
  if [[ $# -gt 0 ]]; then
    result=$(echo "scale=$float_scale; $*" | bc -q 2>/dev/null)
    stat=$?
    if [[ $stat -eq 0  &&  -z "$result" ]]; then 
      stat=1
    fi
  fi
  echo $result
  return $stat
}

# Evaluate a floating point number conditional expression.
function float_cond() {
  local cond=0
  if [[ $# -gt 0 ]]; then
    cond=$(echo "$*" | bc -q 2>/dev/null)
    if [[ -z "$cond" ]]; then 
      cond=0   
    fi
    if [[ "$cond" != 0  &&  "$cond" != 1 ]]; then 
      cond=0
    fi
  fi
  local stat=$((cond == 0))
  return $stat
}
###########################################################
# Floating point number functions end

#Prefix for images
FILE_NAME=${1%.*}
#echo $FILE_NAME

NUMBER_OF_THUMBNAILS=9
MOVIE="$1"
COUNTER=0

#Number of seconds
MOVIE_DURATION=`ffprobe -show_entries "format=duration" -of "default=nokey=1:noprint_wrappers=1" -i $MOVIE 2> /dev/null`
echo $MOVIE_DURATION

MOVIE_WIDTH=`ffprobe -select_streams v:0 -show_format -show_streams $MOVIE 2> /dev/null | grep "^width=" | sed -e "s/width=//"`
MOVIE_HEIGHT=`ffprobe -select_streams v:0 -show_format -show_streams $MOVIE 2> /dev/null | grep "^height=" | sed -e "s/height=//"`
echo "$MOVIE_WIDTH x $MOVIE_HEIGHT"

TW=`float_eval "$MOVIE_WIDTH/3"`
TH=`float_eval "$MOVIE_HEIGHT/3"`

THUMB_WIDTH=${TW%.*}
THUMB_HEIGHT=${TH%.*}
#echo "$THUMB_WIDTH x $THUMB_HEIGHT"

for i in `seq $NUMBER_OF_THUMBNAILS`
do
  let COUNTER=COUNTER+1
  #echo $COUNTER
  
  LOCATION_FLOAT=`float_eval "($i-0.5)*$MOVIE_DURATION/$NUMBER_OF_THUMBNAILS"`
  #echo $LOCATION_FLOAT
  LOCATION_INT=${LOCATION_FLOAT%.*}
  #echo $LOCATION_INT
  
  # Create the thumbnails
  ffmpeg -y -i $MOVIE -ss $LOCATION_INT -vf select='eq(pict_type\,I)' -vframes 1 -s `echo $THUMB_WIDTH`x`echo $THUMB_HEIGHT`  "`echo $FILE_NAME`_$COUNTER.jpg"
done

# Create the gallery
montage -density 96 -tile 3x3 -geometry +4+4 -border 1 "`echo $FILE_NAME`*.jpg" "`echo $FILE_NAME`-thumbnail.jpg"

This is an implementation of the pseudocode found at superuser.com.