Most people use Flash when making some type of animation for the web. If your animation is going to be rather basic, you could always just make an animated gif.
As said you can use Flash! I personally use Photoshop though because it is a lot simpler for the simple flashing animating banners I like to use
A good free alternative to flash is pencil 2d. I'd give you a link but I have got a high enough ranking to do that yet. Look for it on google.
I agree with kharen, Swishmax is much easier to manipulate. But a combination of flash and swishmax would be great
Well, if you want to animate 3d models dont use image ready, if you want to animate banners for example swishing in and out, image ready is right for you, it's not very advanced.