I had been wanting to find a way for NextGen to Automatically scan for new images rather than doing this manually. Couldn't find anything available and noticed there is some interest expressed by several others for this same option, so I had a plugin created that does this for you on page refresh. Every time the page containing the gallery is refreshed, the plugin automatically scans for new images not already found in the Database, adds them to the NextGen database and creates thumbnails. In practical function, two page refresh requests are required: one which tells the plugin to go search for new images, and the second to display them... but it works the same for the second refresh if you just click on an image already in the gallery. So if you have a user that loads the page, the request is sent, and then if they click on an image already in the gallery, when they return to the thumbnail or list view, the new images will appear.
Please let me know if you need this solution and I can work with my plugin developer to get it to you.