It should work for your home page. The problem is that I can see that there are 2 differents sets of code running around for the same function...
This is the mouseover on your home page...
Code:
onmouseover="changeImages('nav_pic', '/girlsnight/images/nav_pic-nav_jewelry_over.gif'); return true;" onmouseout="changeImages('nav_pic', '/girlsnight/images/nav_pic.gif'); return true;"
and this is for your jewelry and about pages...
Code:
onmouseover="changeImages('nav_pic', 'img/nav_pic-nav_jewelry_over.gif'); return true;" onmouseout="changeImages('nav_pic', 'img/nav_pic.gif'); return true;"
Notice that they have different image locations (one is img/ and the other is girlsnight/images/), this should also be reflected within the script in the HEAD part as well.
Did you paste the code in between the head tags inside skin1/customer/home.tpl?