       Images can be rendered to RLE and PBMPlus files.  They can
       also be held in pixmaps and displayed using the a Tk photo
       image.   Collectively they are referred to as image store.
       Image store is referenced by handles in various  commands.
       Two attributes affect operations done on image storage, if
       they can be read,  written  or  both  and  their  scanning
       direction (top to botton, botton to top, or random).

       Scanning  direction  determines if image store is accessed
       from top to bottom scan line, from bottom to top  or  ran-
       domly.   File  objects  can only be accessed in one direc-
       tion.

       The following are the  supported  image  store  types  and
       their attributes:

            o RLE files - read/write, bottom to top access.

            o PBMPlus files - readonly, top to bottom access.

            o Tk photo images - read/write, random access.

            o pixmaps - read/write, random access.
