Create custom image attribute in magento

First Create "size_image" attribute type media gallary and set below code

<?php if($_product->getSizeImage() && ($_product->getSizeImage() != 'no_selection')): ?>
        <div class="rightimage">
                <img src="<?php echo $this->helper('catalog/image')->init($_product, 'size_image');?>">
        </div>
<?php endif; ?>
                        
also set its placeholder in this skin directory : images/catalog/product/placeholder/size_image.jpg