Should I add nofollow to banner ads?

If someone buys a text link from my website I add the nofollow tag so you know it’s a paid link. Should I do the same for banner images also? Damian, Blogosphere Russian subtitles provided by Mike Shakin. Have a question? Ask it in our Webmaster Help Forum: www.google.com Want your question to be answered on a video like this? Follow us on Twitter and look for an announcement when we take new questions: twitter.com More videos: www.youtube.com Webmaster Central Blog: googlewebmastercentral.blogspot.com Webmaster Central: www.google.com