Using Microsoft Photo Editor to Change the Transparency of a GIF image

Open the image.  If the image already has transparent areas, they will be shown as a checkerboard pattern.

To remove the current transparency, right click the image and select 'Properties'.

Uncheck the 'Transparency' box.  Click 'OK'.

Transparency settings have now been removed from the image.  The checkerboard pattern is no longer shown.

Now make a new transparency setting.  Using the pull down menu (or the Transparency Icon), pick the desired transparent color(s).

The cursor changes to a 'Pick' tool.  For this example, the original white background is selected.  Click 'OK'.  Note that a range of colors or a degree of transparency can also be selected.

The background has returned to a checkerboard pattern, indicating that it is now set to be transparent.

Save the file as a GIF and you're done!