Edit you apache httpd.conf and add a <Directory...> directive like this:
Code:
<Directory "/apache/docroot/pictures">
Options Indexes FollowSymLinks
Order allow,deny
Allow from all
</Directory>
Where /apache/docroot is the DocumentRoot of your server.
The keyword is "Indexes" which tells apache to index the contents of the directory specified by the directive. Of course you can just add "Indexes" inside the directive of your DocumentRoot directory to set this globally.