I'm sorry to say, but there is more going on behind the scenes with magento images that it simply putting a filename in the database and linking to your image. The cache generation alone is pretty complex. I believe you are going to have a hard time doing it the way you are attempting.
That being said, I do have a suggestion. Since your images are already on the server, I suggest you write a simple php script to tell magento to attach them to the product image. This could be automated, but i'll give you a small example below...
To attach an image, you would simply navigate to the url like this
http://yoursite.com/imageattacher.php?sku=YOURSKU&image=yourimagefilename.jpg
The script would be like this... create a file in your magento ROOT and call it imageattacher.php. Upload your images to the magento media import directory. I have not tested this but it should work.
<?php
// Initialize magento for use outside of core
umask(0);
require_once 'app/Mage.php';
Mage::app('admin');
// Get the variables from the URL
$sku = $_GET["sku"];
$imageName = $_GET["image"];
// get the image from the import dir
$import = Mage::getBaseDir('media') . DS . 'import/' . $imageName;
// Load the product by sku
$product = Mage::getModel('catalog/product')->loadByAttribute('sku',$sku);
// if the product exists, attempt to add the image to it for all three items
if ($product->getId() > 0)
{
// Add the images and set them for their respective usage (the radio button in admin)
$product->addImageToMediaGallery($import,array('image', 'small_image', 'thumbnail'),false,false);
// Make the changes stick
$product->save();
}
?>
See this http://www.danneh.org/2012/05/creating-products-programmatically-magento/