This is the code provided by Dreamweaver CS3 when a simple mouseover/hotlink is created. I hope it helps.
<body onload="MM_preloadImages('
path to hidden image')">
<a href="
path to URL if this is a hotlink too otherwise #" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('Image1','','
path to hidden image',1)"><img src="
path to first visible image" name="Image1" width="
x" height="
y" border="
z" id="Image1" /></a>
x and y is the dimension of a rectangular area that the mouse arrow must be within in for the mouseover to work.