Assign first media image as base image.

assign_baseimage.php

<?php
include 'app/Mage.php';
Mage::app();

$productId = 4299;
//load the product
$product = Mage::getModel('catalog/product')->load($productId);
//get all images
$mediaGallery = $product->getMediaGallery();
//if there are images
if (isset($mediaGallery['images'])){
    //loop through the images
    foreach ($mediaGallery['images'] as $image){
        //set the first image as the base image
        Mage::getSingleton('catalog/product_action')->updateAttributes(array($product->getId()), array('image'=>$image['file']), 0);
        //stop
        break;
    }
}