If it is an image, saving it as an avi will result in a faulty file. Try opening it in winamp or whatever video player you use, chances are you'll get an error message.
You should simply load it into the bump channel, and animate the whole material in the advanced material editor.
Or create a second heighfield, and use this 2nd map at some point in your bump channel. If  you have already set up an animation, Vue will ask if you want to animate the transition between the two bump functions. Say yes, and the bumps will morph into one another.