Templates
The most important feature improvement in Version 1.00 is the template engine. Custom templates are PHP files that can be stored in the folder nggallery, inside your theme directory. NextGEN gallery look up always first into this folder if there are the vaild template file. For example, if you are using the default WordPress theme then you can store a template file at :
/wp-content/themes/default/nggallery/gallery-sample1.php
To use now this template in a post or page you insert the tag:
[nggallery id=1 template=sample1]
This tells NextGEN Gallery to use ‘gallery-sample1.php’ to show the gallery output. Without this template tag, NextGEN takes the default template (gallery.php) from the plugin folder (unless you copy it to your theme folder), normally located at :
/wp-content/plugins/nextgen-gallery/view/gallery.php
The same you can use for album, singlepic or imagebrowser, all of them are template driver so that you can easily modify the output without having problems during a upgrade.
Resource: http://nextgen-gallery.com/templates/