Javascript: Changing css attributes

[VaporAction]

'm new to javascript so my code is probably all messed up...anyway, here goes...
I'm trying to make a drop down menu composed of images using javascript...
I tried making function that would be triggered by onMouseOver and onMouseOut events...
I know how to make it happen if I use a different function for each menu item...but for efficiency I tried writing the function so that it would know which menu item had called it and display the appropriate image...just for testing purposes, I'm only using 2 menu items right now...they're images with the id "topChem" and "topMin"

"topChem" when moused over should display "botChem"

"topMin" when moused over should display "botMin"

here's the code that hasn't been working...

function menuOver() {
	var source = window.event.srcElement.id;
	if (source==document.getElementById("topChem")) {
		document.getElementById("botChem").style.visibility = "visible";
	}
	else if (source==document.getElementById("topMin")) {
		document.getElementById("botMin").style.visibility = "visible";
	}
}