How to add gallery slug to attachment url?

Question

I am using standard WordPress gallery, but I am not happy with the url to image (link to attachment page).

In my current situation gallery url is:

http://example.com/gallery/

and image url is

http://example.com/image01/

What I am trying to achieve is that gallery url will remain the same, but image (attachment) url will be:

http://example.com/gallery/image01/

I can customize my gallery code to add the gallery slug to attachment url (probably by creating custom shortcode which will get content of gallery shortcode and just replace urls), but I am not able to make .htaccess file to work (if is there better way then using .htaccess file, I don't mind that).

My current .htaccess code is

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /main/dmm/wp01/


# beginning of my code
RewriteRule ^selected-work/(.*)/ $1/
# end of my code


RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /main/dmm/wp01/index.php [L]
</IfModule>
# END WordPress

# Caching disabled    
<filesMatch "\.(html|htm|js|css)$">
  FileETag None
  <ifModule mod_headers.c>
     Header unset ETag
     Header set Cache-Control "max-age=0, no-cache, no-store, must-revalidate"
     Header set Pragma "no-cache"
     Header set Expires "Wed, 11 Jan 1984 05:00:00 GMT"
  </ifModule>
</filesMatch>    
# END OF Caching disabled

Answer 1

So I was able to make it work (without .htaccess as Tom sugested) using WordPress Rewrite API. Here is my working code (from functions.php):

function add_rewrite_rules( $wp_rewrite )
{
    $new_rules = array
    (
        '(the-gallery-slug)\/(.?.+?)(?:/([0-9]+))?/?$' => 'index.php?pagename=$matches[2]'
    );

    // Always add your rules to the top, to make sure your rules have priority
    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}

add_action('generate_rewrite_rules', 'add_rewrite_rules');

And here are links that helped me a lot:

"Wordpress - wp_rewrite rules" post on stackoverflow

Debug WordPress 404 issues (permalinks, rewrite rules, etc.)