I've got some suggestions for this plugin, to clean up the code and reduce on database action etc. Could you drop me a line? :)

Cheers, Joost

In the meanwhile why not switch your RSS to RSS2, it is the newer standard for RSS I believe, honestly RSS isn't my strongest suit!

if(isset($_REQUEST['feed_logo'])){
    $logo_filename = $_REQUEST['feed_logo'];
    update_option('feed_logo', $logo_filename);
}

should read

if(isset($_REQUEST['feed_image'])){
    $logo_filename = $_REQUEST['feed_image'];
    update_option('feed_logo', $logo_filename);
}

Alternatively, you could change

    <label for="feed_icon"><strong>Feed Logo:</strong></label> 
        <?php echo get_bloginfo('url'); ?>/<input type name="feed_image" value="<?php echo $default_logo; ?>" size="35" />
        <small><em>Use an image file (jpg, png, gif)</em></small>

to

    <label for="feed_icon"><strong>Feed Logo:</strong></label> 
        <?php echo get_bloginfo('url'); ?>/<input type name="feed_logo" value="<?php echo $default_logo; ?>" size="35" />
        <small><em>Use an image file (jpg, png, gif)</em></small>

which is probably preferable for troubleshooting reasons, as you keep the names consistent.

Also, you might consider dropping the auto-path and just pre-popping a text-field with the current wordpress root, so users can put in a full URL instead of just a path under the wordpress root. My wordpress root is not the same as my web root, and for organizational reasons, it sometimes makes more sense to not dupe items into my wordpress install dir.