How can I change image when clicked.

[wxdqz]

Hi,

How can I change the image plus.gif to say minus.gif when plus.gif is clicked. Please see parent/child structure menu structure below.

<div><img src=http://www.webdeveloper.com/forum/archive/index.php/"plus.gif">Main Link1
<ul id=menu_1" style="display:none">
<li>Link 1</li>
<li>Link 2</li>
</div>
<div><img src="plus.gif">Main Link2
<ul id=menu_2" style="display:none">
<li>Link 1</li>
<li>Link 2</li>
</div>