Filter Magento Product Collection for Products with Images

I have a product collection that I am getting from a configurable product, and I want to filter that collection to only include products that have images attached to them. How do I go about th开发者_高级运维is?

Code looks something like this:

$configurableProduct = $this->getConfigurableProduct();
$childProducts = $configurableProduct->getTypeInstance()->getUsedProductCollection();

// add some filter here to get only products with images
$childProducts->addAttributeToFilter( /* what goes here? */ );    

foreach( $childProducts as $product ) {
   ...
}

Any help is appreciated. Thanks! Joe

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$this->_productCollection->addAttributeToFilter('small_image',array('notnull'=>'','neq'=>'no_selection'))

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Works also with this:

$collection->addAttributeToFilter('small_image',array('notnull'=>'','neq'=>'no_selection'));