I figured it out. Using the Genesis framework, you can do this, adapted from http://dreamwhisperdesigns.com/genesis-tutorials/genesis-default-thumbnails/
/**
* Default Image
*
* @author Jen Baumann
* @link http://dreamwhisperdesigns.com/?p=429
*/
add_filter('genesis_get_image', 'dream_default_image', 10, 2);
function dream_default_image($output, $args) {
global $post;
$slug = $post->post_name;
$thumbnail="http://foodiefun.imgix.net/featured/".$slug.'.jpg?fit=fill&bg=f9f9f9&w=400&h=600';
switch($args['format']) {
case 'html' :
return '<img src="'.$thumbnail.'" class="alignleft post-image" alt="'. get_the_title($post->ID) .'" />';
break;
case 'url' :
return $thumbnail;
break;
default :
return $output;
break;
}
}