neato-content-filter.php is a sample filter for the_content in WordPress

neato-content-filter.php

add_filter( 'the_content', 'neato_content_filter', 20 );
/**
 * Add a spinning icon to the beginning of every post page.
 *
 * @uses is_single()
 */
function neato_content_filter( $content ) {

	if ( is_page() ) {
		// Add image to the beginning of each page
		$content = sprintf(
			'<img class="post-icon" src="%s/images/spinner-2x.gif" alt="Post icon" title=""/>%s',
			home_url( 'wp-includes' ),
			$content
		);
	}

	// Returns the content.
	return $content;
}