Easy way, use Open CV.

import cv2

vc = cv2.VideoCapture('Test.mp4')
c=1

if vc.isOpened():
    rval , frame = vc.read()
else:
    rval = False

while rval:
    rval, frame = vc.read()
    cv2.imwrite(str(c) + '.jpg',frame)
    c = c + 1
    cv2.waitKey(1)
vc.release()

At it's most simple, extract single image from frame of video:

ffmpeg -i video.mp4 -ss HH:MM:SS -vframes 1 image.jpg

#!/bin/bash

file="$1"
name="${file%.*}"

# Extract the first frame
ffmpeg -hide_banner -i "$file" -vf "select=eq(n\,0)" -vsync vfr "$name-first_frame.jpg"

# Set the frame interval
frame_interval=30

# Calculate digits required for frame numbering
total=$(ffmpeg -i "$file" -map 0:v:0 -c copy -f null -y /dev/null 2>&1 | grep -Eo 'frame= *[0-9]+' | grep -Eo '[0-9]+' | tail -1)
digits=${#total}

# Extract every nth frame and rename to include actual frame number
ffmpeg -hide_banner -i "$file" -vf "select='not(mod(n\,$frame_interval))',showinfo" -vsync vfr temp_%0${digits}d.jpg 2>&1 | grep 'pts_time' | awk -v name="$name" -v digits="$digits" -F'|' '{ 
    split($5, a, "="); 
    split($8, b, "="); 
    printf "mv temp_%0" digits "d.jpg " name "-frame_%0" digits "d.jpg\n", b[2], b[2]*30 
}' | bash

# Remove temporary files
rm temp_*.jpg

exit 0

Save as: frame_extract.sh
Change mode executable: chmod +x frame_extract.sh
Usage: ./frame_extract.sh input_file.mkv

# Define the file path as a parameter
param(
    [Parameter(Mandatory=$true)]
    [string]$FilePath
)

# Get the file name without extension
$name = [System.IO.Path]::GetFileNameWithoutExtension($FilePath)

# Extract the first frame
& ffmpeg -hide_banner -i $FilePath -vf "select=eq(n\,0)" -vsync vfr "$name-first_frame.jpg"

# Set the frame interval
$frameInterval = 30

# Calculate digits required for frame numbering with zero padding
$totalFramesOutput = & ffmpeg -i $FilePath -hide_banner -map 0:v:0 -c copy -f null - 2>&1
$totalFrames = ($totalFramesOutput | Select-String -Pattern 'frame=\s*[0-9]+' | ForEach-Object { $_.Matches } | ForEach-Object { $_.Value.TrimStart("frame=") } | Select-Object -Last 1)
$digits = $totalFrames.Length

# Extract every nth frame and rename to include actual frame number
$output = & ffmpeg -hide_banner -i $FilePath -vf "select='not(mod(n\,$frameInterval))',showinfo" -vsync vfr "temp_%0${digits}d.jpg" 2>&1
$output | Select-String -Pattern 'pts_time' | ForEach-Object {
    if ($_ -match 'pts_time:(\d+)') {
        $frameNumber = int
        $paddedFrameNumber = $frameNumber.ToString("D$digits")
        $tempFileName = "temp_" + $frameNumber.ToString("D$digits") + ".jpg"
        $newFileName = "$name-frame_$paddedFrameNumber.jpg"
        Rename-Item -Path $tempFileName -NewName $newFileName
    }
}

# Remove temporary files
Remove-Item "temp_*.jpg"

I'm not the greatest at PowerShell, but this should be close.
I have no way to test it, but it should help. (Hopefully!)

Added logic to grab the first frame.
Output filename uses frame count, not sequential numbers.

Came across this question, so here's a quick comparison. Compare these two different ways to extract one frame per minute from a video 38m07s long:

time ffmpeg -i input.mp4 -filter:v fps=fps=1/60 ffmpeg_%0d.bmp

1m36.029s

This takes long because ffmpeg parses the entire video file to get the desired frames.

time for i in {0..39} ; do ffmpeg -accurate_seek -ss `echo $i*60.0 | bc` -i input.mp4   -frames:v 1 period_down_$i.bmp ; done

0m4.689s

This is about 20 times faster. We use fast seeking to go to the desired time index and extract a frame, then call ffmpeg several times for every time index. Note that -accurate_seek is the default , and make sure you add -ss before the input video -i option.

Note that it's better to use -filter:v -fps=fps=... instead of -r as the latter may be inaccurate. Although the ticket is marked as fixed, I still did experience some issues, so better play it safe.