Hum ...
// Step 1
In rss.php file,
find:
'title' => format_rss_text($row['image_name']),
add after:
'image' => get_thumbnail_code($row['image_media_file'], $row['image_thumb_file'], $row['image_id'], $row['cat_id'], format_rss_text($row['image_name']), ((isset($mode) && !empty($mode)) ? $mode : ""), 1),
find:
'item_title' => $item['title'],
add after:
'item_image' => $item['image'],
// Step 2
In templates/your_template/rss_item.xml file,
find:
<title>{item_title}</title>
add after:
<image>{item_image}</image>
Is work ?