GIF (short for Graphics Interchange Format) images are a good choice for saving animations to use in web pages. They have a ".gif" extension.
GIF images are the only type supported by Opacity that can have animations. In a GIF animation, a series of frames can be saved in the same image and will be shown as an animation when displayed in most web browsers. GIF images are also a good alternative for PNG images if you are worried about the transparency problems with Internet Explorer's PNG rendering.
Note: GIF images only support 1-bit transparency. If you need better transparency support, use the PNG image format.