How can I find all products without images in Magento?

//this builds a collection that's analagous to
//select * from products where image = 'no_selection'
$products = Mage::getModel('catalog/product')->getCollection()->addAttributeToSelect('*')->addAttributeToFilter('visibility', 4)->addAttributeToFilter('status', 1)->addAttributeToFilter('image', 'no_selection');

foreach($products as $product)
{
    echo  $product->getSku() . " has no image \n<br />\n";
    //var_dump($product->getData()); //uncomment to see all product attributes
                                     //remove ->addAttributeToFilter('image', 'no_selection');
                                     //from above to see all images and get an idea of
                                     //the things you may query for
}