Creating .gif on the fly.

Please suggest some component or application which helps creating .gif files. For example, when I want to superimpose one .gif file on to the other, I should be able to provide some parameters to the component and it should create a .gif file which shows the merged one .gif file on top of the other.

I don't have any solutions but I am very interseted in saving images as GIF files. How do you generate those?

Thanks!