Welcome to Pixelation!
First of all, it should be noted that there is no such thing as an animated bitmap. Animated sprites are usually in .gif form (Although there are other types).
If you are looking for a .gif animator, you should check out the "Tools, Resources and Linkage" thread:
http://www.wayofthepixel.net/pixelation/index.php?topic=3467.0There are several there, but here are the ones that I've tried:
-GameMaker: Not built for animating sprites, but it does have a very basic built-in editor. The downside is that you can't alter properties such as speed and looping.
-Take ONE: I found it sort of cumbersome, but it covers pretty much everything animation-wise.
-GraphicsGale: Better than the previous two, but if you want to animate anything as a .gif you have to pay for the full version.
I recommend that you just surf through a few of the programs on the resource thread until you find one that you're comfortable with.