using the script/command giftrans

giftrans is used to make one of the colors in a GIF image ``transparent'', so that on web pages, the background color or image of the web page will show through the transparent areas of the GIF image.

Commonly, white is made transparent:

giftrans -t \#ffffff orig.gif new.gif

giftrans -help


  Usage: giftrans [ -t #rrggbb ] [ -o newfile.gif ] [-l] oldfile.gif

  Convert any GIF file into a GIF89a, making the specified:
        color `transparent'.  The color is specified with the -t option
        followed by #rrggbb, where `rrggbb' are six hex digits which
        indicate the Red-Green-Blue components.
        The default color is white = #ffffff, i.e. if the
        -t option is not specified, `-t #ffffff' is assumed.
        Output goes to stdout unless an -o output file is
        specified.  The -l option lists the GIF file's color table.

        OPTIONS:
          -t Specify the transparent color
          -o Redirect stdout to a file
          -l Only list the color table