[jQuery] Show image gradually

Is possible to add a css image with jquery and show the image
gradually? (for example sliding up and down)
I need something like this menu: www.antena1.ro, without macromedia
flash.
I try to add a css image when mouse hover the menu:
css('backgroundImage', 'url(images/menubg.gif').
But what I need to do to show this image gradually ? Thanks!