I've tried doing it like this...
$element->removeDecorator( 'File');
$decorator = new Cms_Form_Decorator_ImageUpload;
// add to front of decorators
$decorators = $element->getDecorators();
array_unshift( $decorators, $decorator );
$element->setDecorators( $decorators );
But an exception get's thrown in Zend_Form_Element_File "file decorator found... unable to render file element"