You can't share product images on Facebook because this theme uses "background-image" to display product images. It helps to hide bad looking white strips generated by Opencart.
But if you really want to share your images on Facebook then you should open following file:
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catalog/view/theme/vanityshop/template/product/product.tpl
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and replace:
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<a href="<?php echo $popup; ?>" class="colorbox" style="display:block; background-image:url('<?php echo $thumb; ?>'); background-position:center center;"></a>
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for:
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<a href="<?php echo $popup; ?>" class="colorbox"><img src="<?php echo $thumb; ?>" title="<?php echo $heading_title; ?>" alt="<?php echo $heading_title; ?>" id="image" /></a>
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Additionally you can replace following code too:
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<a href="<?php echo $image['popup']; ?>" class="colorbox" style="display:block; background-image:url('<?php echo $image['thumb']; ?>'); background-position:center center;"></a>
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for:
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<a href="<?php echo $image['popup']; ?>" title="<?php echo $heading_title; ?>"><img src="<?php echo $image['thumb']; ?>" title="<?php echo $heading_title; ?>" alt="<?php echo $heading_title; ?>" /></a>
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