How to avoid black areas in GIF-animations?

[MobileVisuals]

I am creating animated wallpaper with JASC animations shop according to my customers specification. When the wallpapers shall have the same width and height, there is no problem.
But when it is specified that the width and height shall be different, black areas appear. JASC says that this is because the source images have an aspect ratio different from the animation. This is true, because the source images have the same width and height.

But how do I avoid the black areas? I see no function in JASC to remove them. Do I have to rescale the source images so that they have the same aspect ratio as the animation? This would be very time consuming.