alt text trouble

[Wordpress 2.51/ Cutline-3 Column Split.] I’m using Ad Rotator Plugin to rotate my header images each time a visitor navigates to
a new page/post. It works beautifully. (See home page Here.) The only
problem is that the images do not have alt_text.

The code I’m using to rotate the images is this:

<div id="header_img">
		<img src="http://spaghettiboxkids.com/blog/wp-content/themes/cutline-3-column-split-11/images/header_<?php echo(rand(1,9)); ?>.jpg"
width="970" height="140" alt="image_alt_text" />
	</div>

The images are located in my theme’s image folder. They’re named header_1.jpg, header_2.jpg, etc, all the way to header_9.jpg. (I uploaded them using ftp client server.)

My question is, how can I give the images alt_text? If these images were in a post or sidebar, I would use …a href…to name them and put the alt_text there. But the rotator code above doesn’t do that, and I’m not sure how to handle the code in order to insert alt_text, or if there’s some other way to go about it?