Magento Set default image from product images

Get filename from here
<?php
$_product = Mage::getModel('catalog/product')->load($ProductId)->getMediaGalleryImages();
foreach ($_product as $value) {
     if($value->getFile() == $yourfilename){
        $newimage = $value->getFile(); //get your file you want to set
     }
}
?>
Now set your default image
<?php
$objprod=Mage::getModel('catalog/product')->load($ProductId);
$objprod->setSmallImage($newimage); //newimage is filename you want to set
$objprod->setImage($newimage);
$objprod->setThumbnail($newimage);
$objprod->save();
?>