Yes, you can "hook" into the RSS feed and call 1 attachment (thumbmail)
Add this line to your functions.php file...
This example will add 3 thumbs, but simply change the "numberposts" to your desired amount...
/* Adding 3 thumbs to the top of the post on RSS */
function rss_custom_format($content) {
//prob uneccessary but checking that it is in fact a feed
if(is_feed()){
global $post;
if(!$post->ID){return;}
$args = array(
'post_type' => 'attachment',
'numberposts' => 3,
'orderby' => 'menu_order ID',
'post_parent' => $post->ID,
'post_mime_type' => 'image',
'order' => 'ASC'
);
$attachments = get_posts($args);
if($attachments){
$thumb = '';
foreach($attachments as $attachment){
$thumb .= wp_get_attachment_link($attachment->ID,'thumbnail','true').' ';
}
}
}
$thumb .= '<div style="clear:both; width:100%;"></div>';
$content = preg_replace('`\[[^\]]*\]`','',$content);
$more = '<a href="'.get_permalink($post->ID).'">Read More ...</a>';
return $thumb.$content.$more;
}
add_filter('the_content_rss','rss_custom_format');
add_filter('the_excerpt_rss','rss_custom_format');