Get all images id from post_content.

new_gist_file.php

function insertedImage_save_meta($post_id) {
		$upPost = get_post($post_id);

		$rawPostDoc = new DOMDocument();
		@$rawPostDoc->loadHTML($upPost->post_content);                     
		$imgs = $rawPostDoc->getElementsByTagName('img');

		foreach($imgs as $img){
				$imgIDStr = substr($img->getAttribute('class'), (stripos($img->getAttribute('class'),'wp-image-')+9), 8);
				$imgID = preg_replace("[^0-9]", '', $imgIDStr);

				if($imgID !== false && $imgID !== '') { // double falsy check because of specific nature of stripos() returns coupled with the preg_replace return. Not sure if this is necessary.
					$imgIDs[] = $imgID;					
				}

		}
		return $imgIDs;
}