First open
www.yoursite.com\components\com_virtuemart\themes\default\templates\product_details\flypage.tpl.php
find the line:
<?php echo $product_image ?>
and change it with this one:
<a href=”<?php echo $mosConfig_live_site; ?>/components/com_virtuemart/shop_image/product/<?php echo $product_full_image; ?>” target=”_blank”>
<img src= ” <?php echo $mosConfig_live_site; ?>/components/com_virtuemart/shop_image/product/<?php echo $product_full_image; ?>” class=”fullvmimage” >
</a>
then style the class=”fullvmimage” somewhere in the css files