<block type=”page/html_breadcrumbs” name=”breadcrumbs” as=”breadcrumbs”/>
Add your new code (e.g.)
<block type=”page/html_lookbookimage” name=”lookbookimage” as=”lookbookimage”/>
Now, to to /app/code/local/Mage/Page/Block/Html and create a new file called Lookbookimage.php with this code inside:
class Mage_Page_Block_Html_Lookbookimage extends Mage_Core_Block_Template
Now create a file, called lookbookimage.phtml in /app/design/frontend/default/YOUR TEMPLATE/template/page/html/
I assume all three steps are required and the names (highlighted) need to match.