image hover

Hi,
Is it possible to have the image grey out slightly upon hover (like it does on the jQuery Pin It Button For Images plugin)? Perhaps it is something that can be done via css?
I have used this plugin before, and I used this code:

.frizzly-image-hover { opacity: 0.5; }

But now, I have updated the site to WordPress 7.3 and the latest version of the plugin but the hover does not show.

How can I solve this problem?

Thank you!