Function Change() - On Link Click, Image Changes From One To Another.?

I found it difficult trying to put a short and concise title for you all to understand.My problem: Basically, I have an image (its a + sign), when clicked I need this + to change to a -. When clicked again, it needs to return to being a +.Below is the current code I am using:

<script type="text/javascript">
imgs=Array("positive.PNG","negative.PNG");
var x=0;[code]....

This code works fine if I just have one instance of it. But If I want multiple ones, one stops working and the other functions instead.
I tried the following:

<p><a href="#"><img src="positive.PNG" alt="" onmousedown="change()" id="test" border=0></a></p>
<p><a href="#"><img src="positive.PNG" alt="" onmousedown="change()" id="test" border=0></a></p>

This causes the problem I explained above.I also tried specifying an id in the function change() syntax, but I couldn't get that to work.