If you look in the root of your PHPNuke directory on your webserver you will find a folder called THEMES. Open up the folder and look for the name of the theme you are currently using (it might be something like Anagram or Odyssey for example). Click on that folder to open it up and find a folder called IMAGES and open that up. In there you will find a file called logo.gif and you can replace that with anything you like as long as it is called logo.gif. Try using something like Xara Webstyle 3 to create an animated gif and then upload that.