That should be a little complicated, as that sprite animation (you're talking about the four-frames animation at th center bottom, right?) is defined within Window class and could only be changed from there. What you could do is just make it transparent sprites in the windowskin and draw it completely anew from Window_Message, which would probably be the easiest way. Depending on what you really want (your post wasn't exactly the most elaborative), there might be an easier solution. Either way, this definately belongs to Script Support.