Just look at this page and the video on it and all will be explained.
You simply need to have the two images setup with the one to be shown when the mouse moves over setup as hidden on load.
Then you can simply use the onMouseIn event handler to hide the existing picture with a fade out effect and to show the hidden picture with a fade in effect option.
Obviously, you will need to do a onMouseOut event handler too if you want the images to revert back to the original way.