O_O An animation made solely with paint? I'm not sure that's possible, even with some newer version I'm yet to use.
I'd recommend something like Adobe Flash as it tends to be what's most widely used for web applications...
And if you're cheap and don't like having to pay good money for your animation toy just google 'Flash clones' or something as I've always managed to stumble upon a free alternative that did half the stuff Flash did. It won't get you the same results though unfortunately.
EDIT: Also if you like to make your animations as a frame by frame gif (you can even use any version of paint) I recommend this little application. It's what I used to make a few animated sigs back in the day...
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