Using Gimp To Make An Avatar

[Dakurai]

Ok I'm trying to make an avatar for Xat for some of you that know about Xat you know what I'm talking about .. Anyway I'm trying to make an animated Pokemon avatar, I'm using the Pokemon Black and White animations for this. So I'm at a point where I have all my frames for the animation but the background is white and I want to remove it and make it transparent but the problem is when I use the color picker on gimp it also picks up the white that is on the Pokemon and if remove it I will ruin the animation. So I'm wondering how do I add a background color to the animated sprite so it can be easily removed when I get my frames.

The steps I suppose to do are..
Open the animated gif in gimp
Crop the area I want
Then Save It As An Animation
Then Use The Xat Frame Extractor
Then I'm suppose to make the background of the avatar tansparent but the background is white and when I remove it, it effects the picture as well..

So basically I want to know how do I add a background color to an animated gif in gimp?

(Sorry if this sounds a bit confusing!)

Any Help Would Be Appreciated!