NextGEN allows you to display a single image. Single Pics will also pop-up using whichever lightbox effect you've selected in the main settings area. Using shortcodes allows you to customize various aspects of the image display. It resizes and modify the picture on the fly (with no manipulation at the orginal picture).
Shortcode format: [ singlepic id=X w=X h=X mode=web20|watermark float=left|right ] without space before/after brackets. In this example, "web20" adds a reflection effect and "watermark" adds a watermark as defined in the settings. Examples below are [ singlepic id=1 w=320 h=240 mode=web20 ] and [ singlepic id=3 w=320 h=240 mode=watermark ].
And a look on the watermark function :