I have an idea. Make your self a sprite, let say 128 pixels high. On that sprite draw a straight line across on the bottom and put Hotspot in the bottom-center.
Like here
<img src="http://dl.dropbox.com/u/34375299/blank1.jpg" border="0" />
Copy that frame as many times as you need.
Draw your characters so the feet of your character are always touching green line.
Like here
<img src="http://dl.dropbox.com/u/34375299/blank2.jpg" border="0" />
Follow that rule for every frame for your animation.