WordPress: Get image id from WordPress post. Images first added to the media uploader and then added to a post aren't attached i.e. you can't use $attachments = get_posts( array( 'post_type' => 'attachment', 'posts_per_page' => -1, 'post_status' =>'inherit', 'post_parent' => $post->ID));

get_attachment_id_from_post.php

<?php	

// you need to add the post content to $post_content

preg_match_all('/src=([\'"])?((?(1).*?|\S+))(?(1)\1)/', $post_content, $images);
if (count($images[2]) > 0) {
	foreach ($images[2] as $src) {
		$image_id = pn_get_attachment_id_from_url($src);
		/*
		Do something with the $image_id 
		 */
	}
}

// from: http://philipnewcomer.net/2012/11/get-the-attachment-id-from-an-image-url-in-wordpress/
function pn_get_attachment_id_from_url( $attachment_url = '' ) {
 
	global $wpdb;
	$attachment_id = false;
 
	// If there is no url, return.
	if ( '' == $attachment_url )
		return;
	// Get the upload directory paths
	$upload_dir_paths = wp_upload_dir();
	// Make sure the upload path base directory exists in the attachment URL, to verify that we're working with a media library image
	if ( false !== strpos( $attachment_url, $upload_dir_paths['baseurl'] ) ) {
		// If this is the URL of an auto-generated thumbnail, get the URL of the original image
		$attachment_url = preg_replace( '/-\d+x\d+(?=\.(jpg|jpeg|png|gif)$)/i', '', $attachment_url );
		// Remove the upload path base directory from the attachment URL
		$attachment_url = str_replace( $upload_dir_paths['baseurl'] . '/', '', $attachment_url );
		// Finally, run a custom database query to get the attachment ID from the modified attachment URL
		$attachment_id = $wpdb->get_var( $wpdb->prepare( "SELECT wposts.ID FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta WHERE wposts.ID = wpostmeta.post_id AND wpostmeta.meta_key = '_wp_attached_file' AND wpostmeta.meta_value = '%s' AND wposts.post_type = 'attachment'", $attachment_url ) );
	}
	return $attachment_id;
}