Try going to adobes photoshop page. A lot of time they have downloads available for free or small fee that can aid in image production. Another possible solution is to look on cnet.com or downloads.com (mirror sites) for adobe photoshop plug ins, and you will find something to make transparencies.
You can also try the saturation effects from the pull down menus. If that is not working because its a .gif, then transform it into a jpeg, set the transparency, and convert it back to a gif.
Justin
P.S. I am going of an older version ( I think 5 not sure though ) so this may not work for you. But it might.