If it helps anybody, this is what I ended up with:
<div class="buy-from">
<h5><?php echo vmText::_ ('COM_VIRTUEMART_EBOOKLINKS') ?></h5>
<div>
<?php if ( !empty($this->product->customfieldsSorted['amazon']) ) {
foreach($this->product->customfieldsSorted['amazon'] as $theseones){?>
<a href="/<?php echo $theseones->customfield_value ?>" target="_blank"><img src="/images/stories/virtuemart/amazon-shop-link.png" alt="amazon-shop" /></a>
<?php }}?>
<?php if ( !empty($this->product->customfieldsSorted['BNlink']) ) {
foreach($this->product->customfieldsSorted['BNlink'] as $theseones){?>
<a href="/<?php echo $theseones->customfield_value ?>" target="_blank"><img src="/images/stories/virtuemart/BN-shop-link.png" alt="B&N-shop" /></a>
<?php }}?>
<?php if ( !empty($this->product->customfieldsSorted['iBook']) ) {
foreach($this->product->customfieldsSorted['iBook'] as $theseones){?>
<a href="/<?php echo $theseones->customfield_value ?>" target="_blank"><img src="/images/stories/virtuemart/ibook-shop-link.png" alt="iBook-shop" /></a>
<?php }}?>
<?php if ( !empty($this->product->customfieldsSorted['kobo']) ) {
foreach($this->product->customfieldsSorted['kobo'] as $theseones){?>
<a href="/<?php echo $theseones->customfield_value ?>" target="_blank"><img src="/images/stories/virtuemart/kobo-shop-link.png" alt="kobo-shop" /></a>
<?php }}?>
</div>
</div>Works fine and if there is a custom string fields in the layout positions (amazon, BNlink, iBook or kobo) that string becomes the image/logo link. Underpopulated logos/images do not show.