       Images can be rendered to RLE and PBMPlus files.  They can
       also be held in pixmaps and displayed using the  Tk  photo
       widget.  Collectively these media are referred to as Image
       Storage Objects (ISOs).  ISOs are referenced by handles in
       various  commands.   Two attributes affect operations done
       on ISOs, if they can be read, written or  both  and  their
       scanning direction.

       RLE  files and pixmaps can be both read and written.  PBM-
       Plus files can not currently be read.  The Tk photo widget
       can only be written.

       Scanning  direction  determines  if a ISO is accessed from
       top to bottom scan line  or  from  bottom  to  top.   File
       objects  can only be accessed in one direction.  RLE files
       go from bottom to top, while PBMPlus objects go  from  top
       to  bottom.   The  Tk  photo  widget  and  pixmaps  can be
       accessed in either direction.
