fatal error with gravatar plugin.

Hey, I activated the Garvatar plugin on my blog, but when I got commented on a post, it gave me this error message when I went on my page and opened the comment area to read the comments on my page:

Fatal error: Call to undefined function: gravatar_image_link() in /home/www/studnow.com/wp-content/themes/wave/comments.php on line 78

I don’t know what it means. So, I just deactivated the plugin, and it’s fine now, on the site. But, it’s not doing what I want it to do of course.

So, how can I fix this problem so that my gravatar feature works?

Thanks in advanced.