|BEFORE -->||<-- AFTER|
This will invoke the background color selector and masker
dialog box below.
NOTE that the numbering of color indexes begins at 0 NOT at 1.
From the example .GIFs at the top of the page you would click on the bright yellow color index box.
When you have correctly selected the background color index the
display of the .GIF behind the dialog box will snap the non-background
portions of the .GIF file to black. You should see your image in silhouette.
This is a check that allows you to see that you have selected the correct color index for the background.
NOTE: Not all scanners or screen capture programs will produce .GIFs with solid and smooth backgrounds. You may have to load your .GIF file into a graphics editor that allows you to consolidate the background color into a single color index before you can make a good-looking transparent image.
A little experimentation and practice may be required to get it right.