<?php c2c_the_category_image(); ?>

... and you'll get a category-specific image, for summary and/or full-text feeds.

I managed to hack wp-rss2.php to show a static image together with summary syndication. Here is the hacked file so you can see exactly where to insert the edit (discovered by trial-and-error, and with assistance from the 9rules Clubhouse).

If you want a dynamic image you could try inserting a PHP block in its place.

First, just to clarify, you're not taking about the channel image ("feed logo") as per the RSS spec are you?

What kind of image do you want to add? A static one? A dynamic one that reflects the post category, or an ad?

As I understand it, when you choose Summary for syndication feeds in WordPress Admin (Options > Reading) you strip away all HTML tags—including those for embedded images.

It seems that even a plugin like Better Feed, which lets you add custom elements, will only work if you enable full text feeds.

If you used FeedBurner to summarise your full text feeds, you could use Photo Splicer to add an image from Flickr. You could set up an account specifically for this purpose, or selectively tag images in an existing account. Maybe you could even write a custom FeedFlare?!

The only other option I can think of is to try manually hacking your "core" wp-rss2.php file. I had a quick go at this just now and couldn't get an image to display for summaries (but did insert a test image for full text feeds).