In NextGEN Gallery users have the option for using either text or image watermarks. To set up the feature, go to Other Options and then Watermark.
If using text, you can set the position and offset of the watermark and then the size of the font and opacity.
If an image, you can select the position and offset. To use an image simply upload the watermark file (I recommend using a transparent PNG) to your WordPress Media Library. Copy the file's URL and paste it into NextGEN Gallery's Watermark option.
Once set up, go to Manage Galleries, select the galleries you want to watermark and then use the bulk watermark option to finish the process.
Depending on how mange images there are the process could go quick or take some time.
If you have questions please let us know.
Thanks for reading,