I'm using  CreateCommandButton, but want to dynamically change the underlying img in javascript without having to do a call back to the server and reconstructing the button. 

How???

No worries.  I figured it out.  I created two command buttons and then wrote javascript to hide one or the other depending on the state.  The only trick was recognizing that the cute software will prepend a "cmd_" to the ID of the img tag of the command button.

In short, the javascript to hide a button looks something like...

  document.findElementById('cmd_MyButtonCommand').style.display='none';

and to restore

  document.findElementById('cmd_MyButtonCommand').style.display='';

Is there a better way?