gistfile1.php

<?php
function add_img_titles( $content ) {
    global $post;

    preg_match_all( '/<img[^>]*>/', $content, $matches );
    
    $replacements = array();
    
    foreach( $matches[0] as $match ) {
        // If this image has a title, skip it
        if ( preg_match( '/title=\s*/', $match ) ) {
            continue;
        }
        
        $newimg = str_replace( '<img ', '<img title="' . $post->post_title . '" ', $match );
        
        $replacements[ $match ] = $newimg;
    }
    
    foreach( $replacements as $original => $replacement ) {
        $content = str_replace( $original, $replacement, $content );
    }
    
    return $content;
}