If you want to make GIF-images with a transparent background color,
you can determine this about the menu item File -> Save as... . See also: Save as... Please choose a color in Hardcopy as the wanted foreground color.
(If the original image is an image made with more than 256 colors, please change the color resolution to 256 colors.)
As follows the image to insert.
The chosen color for the transparent background is "Green".
In the following example you can see the result.
It is about two GIF-files which are shown here both.