When you create imagefield gallery you can easily use first image to set it as teaser image if you use Views and Fields for row style but if you use NODE than you have a problem. Also with views you usually override or make lists of nodes but for full node display you need to use something else like Panels, or you can also try overriding nodes with views but that's not best practice. After trying Display Suite, Panels etc easiest way to achive this is through Contemplate, you edit your teaser like this
<div class="field field-head-pict">
<?php print '<a href="'.$node->path.'">'. theme('imagecache', 'default_200', $node->field_gallery[0]['filepath']).'</a>'; ?>
</div>
<div class="field field-body">
<?php print $node->content['body']['#value'];?>
</div>
or body output
<div class="field field-head-pict">
<?php print theme('imagecache', 'default_600', $node->field_gallery[0]['filepath']);;?>
</div>
<div class="field field-type-text field-field-headline">
<div class="field-items">
<div class="field-item"><?php print $node->field_headline[0]['view'] ?></div>
</div>
</div>
<div class="field field-body">
<?php print $node->content['body']['#value'];?>
</div>
<div class="field field-type-filefield field-field-gallery">
<div class="field-items">
<?php foreach ((array)$node->field_gallery as $item) { ?>
<div class="field-item"><?php print $item['view'] ?></div>
<?php } ?>
</div>
</div>
and you are done