Ahum... Softimage is basically, as you know, a 3D package, less suitable to animate 2D pictures. To get good results, I suggest the use of Photoshop (making a layer for each important lips movement), AfterEffects, Flash, and so on.
But there might be a way in Softimage, I would like to know what everybody thinks.
Map (UV coordinates) a cat picture on a NURBS grid, and make sure that you have control points near the mouth.
Simply shape-animate those control points, to move the mouth.
The concept is based on the fact that when the textures are UV-mapped on a NURBS surface, the texture follows the deformation of the wireframe. If the wireframe is deformed down, then the neighbouring part of the texture goes down too.
Maybe you could also animate the visibility (Overall Blending in the textures) of differents textures of the cat. So you'll be able to create many expressions for the cat.This technique could be useful to animate all of the facial features.
I hope this solves your problem
Bernard Lebel