Well, in 2.0, add something like this to your wp-rss2.php file:
<image>
<title>YOUR BLOG NAME</title>
<link>YOUR BLOG LINK</link>
<url>YOUR IMAGE URL</url>
</image>
Title should allow text when your image is moused over and is also supposed to act like alt text in the event that the image file is not available, I believe. Providing a link as shown should also make your image clickable in aggregators. It’s probably something very similar in RSS 0.9. If you have a Feedburner account, you can easily activate this service with them and not have to pick through your code…