A quick PHP script to generate a folder of thumbnails from a source folder of JPG image files. PHP must be able to write files to the local directory.

thumbs.php

<?php

// Make sure we see processing errors during development/testing.
error_reporting(E_ALL);
ini_set('display_errors', true);

// Set up operating parameters.
$filePattern = 'gallery/*.[jJ][pP][gG]';
$thumbPattern = 'thumbs/%s';
$targetWidth = 200;
$targetQuality = 80;

// Create the output dir, if it doesn't already exist. This will fail
// if PHP does not have write access to the local folder.
$thumbDir = sprintf($thumbPattern, '');
if (!is_dir($thumbDir)) {
	mkdir($thumbDir);
}

// Abort if GD is not installed.
if (!function_exists('imagejpeg')) {
	die('PHP does not have the GD extension installed. Aborting.');
}

// Abort if the output directory is not writable.
if (!is_writable($thumbDir)) {
	die('The output directory is not writable by PHP. Check permissions. Aborting.');
}

// Loop over all found jpg files.
foreach (new GlobIterator($filePattern) as $f) {

	// var_dump($f->getRealpath()); // Debugging the path name.

	// Skip regenerating cached thumbs for performance.
	$thumbPath = sprintf($thumbPattern, $f->getFilename());
	if (file_exists($thumbPath)) {
		continue;
	}

	// Determine thumbnail output size.
	list($w, $h) = getimagesize($f->getRealpath());
	$ratio = $h / $w;
	$targetHeight = round($targetWidth * $ratio);

	// Generate the thumbnail from the original.
	$jpg = imagecreatefromjpeg($f->getRealpath());
	$thumb = imagecreatetruecolor($targetWidth, $targetHeight);
	$success = imagecopyresampled(
		$thumb,
		$jpg,
		0, 0, 0, 0,
		$targetWidth, $targetHeight, $w, $h
	);
	imagejpeg($thumb, $thumbPath, $targetQuality);
}

See here: https://stackoverflow.com/a/32848242/70876