Cross fade help!

Hi,

I have 3 Buttons

button1, button2 and button3
and three pictures in three layers.
Which are

layer 1: picture1
Layer2: Picture2
layer3: picture3

Now what my client want is on mouse over button1 - it should show only picture1 and once we mouse over button2 , it should cross fade the picture1 and picture2 and eventually it should show picture2 and same things goes for the picture3.

What i believe is all about cross fade. I am dont have any idea how to do that??

Can some one help me with example??