If an image is not a stream of pictures (Local or Scan option in the Open Image From dialog), you can crop it, adjust colours or modify it in any other way in the Retouch Room. You and your picture can move to this room by choosing Image->Edit in the room main menu.
In case you change the size of the picture in the Retouch Room, the Create Room will fit cutting lines to the new size. It is possible that some pieces are made smaller insofar that they may be discarded later.
Related topics: Retouch Room, Minimal size of a piece, Number of pieces