Joomla layouts to generate thumbnails on the fly

templates\protostar\html\layouts\com_content\article\image.php

<?php
/**
 * @package     Joomla.Site
 * @subpackage  Layout
 *
 * @copyright   Copyright (C) 2005 - 2015 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

extract($displayData);

/**
 * Layout variables
 *
 * @var  stdClass   $article  Article as it comes from joomla models/helpers
 * @var  array   $options     Array with options. Available options:
 *       						'resolution'    : Resolution of the thumbnail
 *       						'force_png'     : Convert always to PNG for transparent backgrounds
 *       						'force_rewrite' : Force thumbnail regeneration
 *       						'thumb_method'  : Method to use to create the thumbnails. See JImage
 */

$layoutOptions = empty($options) ? new JInput : new JInput($options);

$images = json_decode($article->images);

// Intro image available
if (!empty($images->image_intro))
{
	$imageUrl = $images->image_intro;
	$imageAlt = $images->image_intro_alt;
}
// Full text image available
elseif (!empty($images->image_fulltext))
{
	$imageUrl = $images->image_fulltext;
	$imageAlt = $images->image_fulltext_alt;
}
// Nothing to display
else
{
	return;
}

$resolution  = $layoutOptions->get('resolution', '350x250');

// A thumbnail has been required
if ($resolution)
{
	$thumbUrl = JLayoutHelper::render(
		'image.thumbnail',
		array(
			'imageUrl' => $imageUrl,
			'options'  => $options
		)
	);

	if ($thumbUrl)
	{
		$imageUrl = $thumbUrl;
	}
}
?>
<img src="<?php echo $imageUrl; ?>" alt="<?php echo $imageAlt; ?>" />

templates\protostar\html\layouts\image\thumbnail.php

<?php
/**
 * @package     Joomla.Site
 * @subpackage  Layout
 *
 * @copyright   Copyright (C) 2005 - 2015 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

extract($displayData);

/**
 * Layout variables
 *
 * @var  array   $imageUrl    Url of the source image
 * @var  array   $options     Array with options. Available options:
 *       						'resolution'   : Resolution of the thumbnail
 *       						'force_png'    : Convert always to PNG for transparent backgrounds
 *       						'force_rewrite' : Force thumbnail regeneration
 *       						'thumb_method' : Method to use to create the thumbnails. See JImage
 */

$layoutOptions = empty($options) ? new JInput : new JInput($options);

// Generate image path from its URL
$imagePath = JPATH_SITE . '/' . ltrim($imageUrl, '/');

// Cannot create the thumb
if (!file_exists($imagePath))
{
	echo $imageUrl;

	return;
}

// Layout options
$resolution   = $layoutOptions->get('resolution', '450x350');
$forcePng     = $layoutOptions->getBoolean('force_png', false);
$rewriteThumb = $layoutOptions->getBoolean('force_rewrite', false);
$thumbMethod  = $layoutOptions->getInt('thumb_method', JImage::SCALE_INSIDE);

$fileInfo =	pathinfo($imagePath);
$thumbPath = dirname($imagePath) . '/thumbs/' . $resolution . '/' . $fileInfo['filename'] . '.png';

// Thumb already exists
if (!$rewriteThumb && file_exists($thumbPath))
{
	echo str_replace(JPATH_SITE, JUri::root(true), $thumbPath);

	return;
}

// Source of the png file if we have to force PNG thumbnails
$pngPath = dirname($imagePath) . '/' . $fileInfo['filename'] . '.png';

if ($forcePng && $fileInfo['extension'] != 'png' && !file_exists($pngPath))
{
	$pngImage = imagecreatefromstring(file_get_contents($imagePath));
	imagepng($pngImage, $pngPath);
	$jimage = new JImage($pngPath);
}
else
{
	$jimage = new JImage($imagePath);
}

try
{
	$thumbs = $jimage->generateThumbs(array($resolution));
}
catch (Exception $e)
{
	$thumbs = array();
}

if (!$thumbs)
{
	echo $imageUrl;

	return;
}

// Try to create the thumb folder
if (!is_dir(dirname($thumbPath)) && !mkdir(dirname($thumbPath), 0755, true))
{
	echo $imageUrl;

	return;
}

$thumbs[0]->toFile($thumbPath, JImage::getImageFileProperties($jimage->getPath())->type);

echo str_replace(JPATH_SITE, JUri::root(true), $thumbPath);

templates\protostar\html\layouts\joomla\content\intro_image.php

<?php
/**
 * @package     Joomla.Site
 * @subpackage  Layout
 *
 * @copyright   Copyright (C) 2005 - 2015 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

// More descriptive name
$article = $displayData;

// Our article will only have a link if it's required so it's easier to check in our HTML
if ($article->params->get('link_titles') && $article->params->get('access-view'))
{
	$link = JRoute::_(ContentHelperRoute::getArticleRoute($article->slug, $article->catid, $article->language));
}
?>
<div class="item-image">
	<?php if ($link) : ?>
		<a href="<?php echo $link; ?>" class="thumbnail">
	<?php endif; ?>
		<?php
			echo JLayoutHelper::render(
				'com_content.article.image',
				array(
					'article'    => $article,
					'options' => array(
						'resolution' => '450x350'
					)
				)
			)
		?>
	<?php if ($link) : ?>
		</a>
	<?php endif; ?>
</div>

Sample code for the article: http://phproberto.com/es/36-create-thumbnails-on-the-fly-for-joomla-content-articles