There must be a restriction for "teaser" and "aftermore" images displayed in RSS feeds. There's currently no limitation, all attached images get displayed, even galleries.

A patch for /skins/_rss2/index.main.php

// Display images that are linked to this post:
$image_position = ($feed_content == 'full') ? '' : 'teaser';
$content = $Item->get_images( array(
		'before' =>              '<div>',
		'before_image' =>        '<div>',
		'before_image_legend' => '<div><i>',
		'after_image_legend' =>  '</i></div>',
		'after_image' =>         '</div>',
		'after' =>               '</div>',
		'image_size' =>          $image_size,
		'restrict_to_image_position' => $image_position,
	), 'htmlbody' );