Anyone know how this works?

As long as you cite it clearly at the end of your work that you got <insert name of said symbol here> from the manga then you'll be fine, especially if you don't intend to take it any higher than this site. If you wanted to say, publish your story then you'd need to work it out with the creator of the manga and the publisher to reach a copywright agreement but, ya, you should be fine long as you tell where you got it :)