Requires ffmpeg and imagemagick

#!/bin/bash

# Check if input video file is provided and exists
if [ $# -lt 1 ]; then
    echo "Usage: $0 <input_video>"
    exit 1
fi

input_video="$1"

if [ ! -f "$input_video" ]; then
    echo "Error: Input video file not found."
    exit 1
fi

# Get video duration in seconds (floating-point)
duration=$(ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "$input_video")

# Check if the duration was successfully extracted
if [ -z "$duration" ]; then
    echo "Error: Could not retrieve video duration."
    exit 1
fi

# Calculate fps as 20 frames divided by the duration
fps=$(echo "20 / $duration" | bc -l)

# Create a directory to store frames
mkdir -p frames


# Extract 20 frames from the video using the fps filter
ffmpeg -i "$input_video" -vf "fps=$fps,scale=200:-1" -frames:v 20 "frames/frame_%02d.jpg" -loglevel error

# Create the collage using ImageMagick's montage tool
montage -mode concatenate -tile 5x4 -geometry +2+2 frames/frame_*.jpg output_collage.jpg

# Clean up temporary files (delete frames directory)
rm -r frames

echo "Collage created: output_collage.jpg"

If you already have the screenshot files, then ImageMagick can create collages from multiple images. I've done that myself too. So that part can be easily automated through scripts.

# program input-files frame input-size output-file
montage shadow*.png -frame 5x4 -geometry 680x160 ./collage.png

In the above command montage is a tool that comes with ImageMagick. First argument is a list of files to as input, here all .png files starting with name shadow*, so it does not load up the existing collage.png file (which is the output). Then you specify the frames, how its ordered for each row. Then the size for the input images. Here all images have the same size. And at last you specify a single filename as output.