Add following code to functions.php:
// THIS INCLUDES THE THUMBNAIL IN OUR RSS FEED
add_filter( 'the_excerpt_rss', 'inoplugs_insertThumbnailRSS' );
add_filter( 'the_content_feed', 'inoplugs_insertThumbnailRSS' );
function inoplugs_insertThumbnailRSS( $content ) {
global $post;
$preview_image = kriesi_post_thumb($post->ID, array('size'=> array('M'),
'display_link' => '_prev_image_link', // '_prev_image_link' or array('lightbox')
'linkurl' => '',
'wh' => $k_option['custom']['imgSize']['M']
));
if ($preview_image){
$content = '<div class="featuredimg">' . $preview_image . '</div>' . $content;
}
return $content;
}