Can I add www.addthis.com button to my zenphoto albums and photos. As you know addthis.com button helps in sharing the contents and have become very popular. Is their any extension or method to add those buttons so that peoples visiting my album can share the album or photos on it.

No plugin and not really necessary. Just add it to your theme pages directly. Please read the theming tutorial.

@acrylian Can you explain the detailed procedure?

Place the code addthis provides on your theme where you want to appear.