NextGEN Gallery allows you to add watermarks to your images individually or as a batch. To set your watermark settings, go to Gallery > Other Options > Watermarks:
HOW TO ADD WATERMARKS TO IMAGES
To add your watermark to your images individually or as a batch, go to Gallery > Manage Galleries > [select gallery to edit] > under bulk actions drop-down select 'Set Watermark'. Click 'Apply' and once the process is complete, be sure to click 'save changes'!