Recently I needed to customise the output of a NextGen Gallery and found a great explanation here.
The steps are as follows:
- Grab a copy of the file gallery.php
- Create a new folder in your current theme directory called nggallery and place your copy of gallery.php in there.
- Rename your gallery.php file to gallery-yourtemplatename.php
- Make your modifications
To use your template in this instance you would use the shortcode [nggallery id=1 template=yourtemplatename] assuming your gallery had an ID of 1.