Filter function for WordPress: this adds an onload attribute onto the img element for calls that use the post_thumbnail_html filter such as get_the_post_thumbnail() or the_post_thumbnail()

filter.thumbnail.functions.php

/**
 *
 * Filter the post thumbnail html adding a javascript onload attribute.
 *
 * @param	html(string)
 * @return	string
*/
function filter_thumbnail_html($html, $attr = false) {
	if (empty($html)) return $html;
	$dom = new DOMDocument();
	$dom->preserveWhiteSpace = false;
	$dom->loadHTML($html);
	$img_tags = $dom->getElementsByTagName('img');
	foreach($img_tags as $img) {
		$img->setAttribute('onload', 'imagine(this)');
		$dom->appendChild($img);
		if ( $attr !== false ) return $img->getAttribute('alt'); 
	}
	return $dom->saveHTML($img);
}
add_filter( 'post_thumbnail_html', 'filter_thumbnail_html', 10, 1 );