Perfect -- thank you!
For future Googlers who may stumble across my post, there are a lot of complex image libraries out there that rank highly when searching, but you can pull this off with built-in PHP functions by first stripping out existing EXIF data by recreating the image like so:
<?php
$path = 'pathtoimage.jpg';
$img = imagecreatefromjpeg ($path);
imagejpeg ($img, $path);
imagedestroy ($img);
?>
Then embedding the new data using the example code here: http://php.net/manual/en/function.iptcembed.php
The line '2#120' => 'Test image' is what corresponds to the image description.