.htaccess rewrite to ignore files

I know this is simple, but I am just not getting the syntax exactly correct:

In my .htaccess file, I have the following rewrite rule:

RewriteRule ^(.+) watermark.php?img=$1 [L]

Code (markup):

This is to display a watermark whenever an image is accessed. This is working.

All of the images have a corresponding thumbnail. For example:


image01.jpg
image01_thumb.jpg

How to I write a rule to ignore any files with "_thumb" in the filename? I don't want a watermark to display on the thumbnails.

Thanks!

 

I know very little about MOD-REWRITE, but wouldn't it be possible to just put the thumbs in a separate folder, and avoid the problem completely?

H@H