Ok, step by step:
You have created a new category, for example the ID is 1.
Now upload your images via FTP to:
data/media/1/image1.jpg
data/media/1/image2.jpg
etc.
and your thumbnails to:
data/thumbnails/1/tn_image1.jpg
data/thumbnails/1/tn_image2.jpg
etc.
Then open the script rename_thumb.php and change
$thumb_prefix = "thumb_";
to
$thumb_prefix = "tn_";
Upload this script to "data/thumbnails/1" and run it. It will remove the prefix tn_ from all your thumbnail files.
Go now to your Control Panel and add the images with Check new images to the database.
Greets Jan