Programmatically add image to product in magento
devm.com.np
<?php
$productId = Mage::getModel('catalog/product')->getIdBySku('sku');
$product = Mage::getModel('catalog/product')->load($productId);
        
$mediaAttribute = array (
                                'thumbnail',
                                'small_image',
                                'image'
                                );


$imageUrl = '';    
$destinationFilepath = '';    

$mediaGalleryAttribute = Mage::getModel('catalog/resource_eav_attribute')->loadByCode('catalog_product', 'media_gallery');

foreach ($product->getMediaGallery('images') as $image) {
       $mediaGalleryAttribute->getBackend()->removeImage($product, $image->getFile());
    }

try{
    $adapterGd2 = new Varien_Image_Adapter_Gd2();
    $adapterGd2->open($imageUrl);
    $adapterGd2->save($destinationFilepath);
    
    $product->addImageToMediaGallery($destinationFilepath, $mediaAttribute, false, false);
    
    unlink($destinationFilepath);    
}
catch(Exception $e)
{
    
}
?>