[Javascript] partially triggering image rollovers

[SoitPP at aol.com]

I've got a website
www.ofviolets.net/~sanspalm

where I'm trying to use image rollovers. 
It's working fine actually.. but only on the bottom halve of the button 
images used to trigger the mouseOver.
I think it might have something to do with the fact that the images are 
positioned, .. perhaps?
Any clues as to why this is happening? I would like the images to trigger if 
the mouse goes over any portion of the image, not just the bottom 1/3 of the 
image.

Peace, 
     Aaron