///мой config.xml

		<blocks>
            <hb_productmedia>
                <class>HB_ProductMedia_Block</class>
             </hb_productmedia>
			<adminhtml>
                    <rewrite>
                        <adminhtml_catalog_product_helper_form_gallery_content>
                            HB_ProductMedia_Block_Adminhtml_Catalog_Product_Helper_Form_Gallery_Content
                        </adminhtml_catalog_product_helper_form_gallery_content>
                    </rewrite>
                </adminhtml>
        </blocks>

////HB/ProductMedia/Block/Adminhtml/Catalog/Product/Helper/Form/Gallery/Content.php

<?php
class HB_ProductMedia_Block_Adminhtml_Catalog_Product_Helper_Form_Gallery_Content extends Mage_Adminhtml_Block_Catalog_Product_Helper_Form_Gallery_Content
{
    public function __construct()
    {
        parent::__construct();
        $this->setTemplate('hb_productmedia/adminhtml/catalog/product/helper/form/gallery/content.phtml');
    }

}
?>

/////app/design/adminhtml/default/default/template/hb_productmedia/adminhtml/catalog/product/helper/form/gallery/content.phtml

<?php
$_block = $this;
var_dump( $_block->getImageTypes());



///Ошибка

Fatal error: Call to a member function setId() on boolean in /Users/vitaliyprokopov/Sites/norwood.dev/app/code/core/Mage/Adminhtml/Block/Catalog/Product/Helper/Form/Gallery.php on line 57