Window > Next This command switches to next opened image.
Window > Previous This command switches to previous opened image.
Window > Close All 
This command closes all opened documents in NIS-Elements.
Window > Close All but Current This command closes all opened documents, but keeps the current one opened.
Window > Close Closes the current image. If the image has been changed or is new, the user will be given a chance to save it.
See Closing Images.
Window > [Opened file] This command brings the selected image into focus (makes it active).