Image name rewrite rule - htaccess

I am trying to add a rewrite rule to my htaccess file which will rewrite an image name based on what has been requested. For example:

<img src="www.site.com/img/intro-image.jpg">

The rule in turn should drop the 'intro-' and get/return image.jpg - or this is what I am trying to achieve.

My rewrite rule is as follows:

RewriteEngine On
RewriteBase /
RewriteRule /img/(.*?)-(.*?).jpg /img/$2.jpg [L]

but this returns a 404?

Can anyone advise? TIA.