Assign default featured image if none input

The default image is:
http://www.clikaclinic.com/images/default-image.jpg
It's ID is 1339.

The form ID is 1.
I'm not sure where to include this information in the code you pointed me to and also like I say not sure in which file to put the code...

Sorry! And thanks again for being so helpful :)

Of course, sorry... being stupid! Have I got this right then?

/*********** Set default featured image for reviews submitted via form ************/
// after form 1 is submitted call function
add_filter("gform_after_submission_1", "gform_set_post_thumbnail");
function gform_set_post_thumbnail($entry){
// get post ID of the created post
$post_id = $entry["post_id"];
// get the last image added to the post
$attachments = get_posts(array('numberposts' => '1', 'post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC'));
if(sizeof($attachments) == 0)
	return; //no images attached to the post
// set image as the post thumbnail
set_post_thumbnail($post_id, 'http://www.clikaclinic.com/images/default-image');
}

Nailed it! Code was (where 1339 is the ID of the default image in my media library):

/*********** Set default featured image for reviews submitted via form ************/

// after form 1 is submitted call function
add_filter("gform_after_submission_1", "addreview_set_post_thumbnail");
function addreview_set_post_thumbnail($entry){
	// get post ID of the created post
	$post_id = $entry["post_id"];
	// get the last image added to the post
	$attachments = get_posts(array('numberposts' => '1', 'post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC'));
	//no images attached to the post
	if(sizeof($attachments) == 0){
		// set image as the post thumbnail
		set_post_thumbnail($post_id, 1339);
	}
}