wanna make some changes in admin's details such as email and login name or password, what can i do?

for admin email, change it on admin panel -> configuration
for password of admin (you I suppose), change it on the customization page when once you're logged as the admin user.

Can this program support AVI or other movie clips?

Not in the official release, but I adapted PhpWebGallery for supporting any files in a version you can d/l to this URL : phpwebgallery.v1.2.documents.zip

MOD for managing "avi" files :

in /include/config.inc.php, line 24, replace

$tab_doc_ext = array( 'doc','DOC', 'pdf', 'PDF' );

by

$tab_doc_ext = array( 'doc','DOC', 'pdf', 'PDF', 'avi', 'AVI' );

for having a thumbnail, you just have to place a file named "avi.gif" in /images/ (like for doc.gif or pdf.gif).