Get a program like animagic, I think photoshop and paintshop do it.. lots of programs out there can make backgrounds transparent.. Your best bet would be to get a gif program or animating program because obviously those will be designed with the ability to make gifs which usually give you a transparent option.
Animagic is what I use, and you can use a paintdropper on the image to select a color to be transparent, which makes it really easy to use.
Just go to
www.download.com and you can find it there along with other gif tools.