What you'll have to do is put two images on top of eachother, then fade the top one out (opacity 1 --> 0), or even fade one out (1 --> 0), and the other in (0 --> 1) so you don't have to worry about stacking order.
Like Ryan said, you can't tween the source of an image. You can only tween properties that can have a transition. With an image source it's either image1 or image2, not a combination of those. -- Tim
