Hello everybody,
i'd like to know if it's possible use zenphoto functions in other pages different from image.php album.php index.php etc. For example i'd like to use some zenphoto function in a page called "a.php" in the "mytheme" folder (where also are stored index.php, album.php and image.php). Hope someone can help me. Thank u so much!

Generally you can use all Zenphoto function on all theme pages (called "custom pages", see the theming tutorial). The image and album functions do only work correctly in context/if the objects are setup. It is possible/necessary to setup those manually using the object class model framework.
http://www.zenphoto.org/documentation/functions/_template-functions.php.html#functionmakeAlbumCurrent
http://www.zenphoto.org/documentation/functions/_template-functions.php.html#functionmakeImageCurrent
See the functions documentation for more details on the class methods.