Adding the WordPress Plugin Search Results Image. Plugin Icon ?

Hi, when one looks for a plugin in the WordPress repository, you can see little thumbnails beside the plugins. I would like to know how to add one to our own plugin result pages. We provide plugins for WP users to install our forms and have the forms submit data to our lead management and third party integration system. ( http://www.contactus.com )

This is what im talking about: http://i.imgur.com/j7HBkPQ.png

And you can find live examples here: http://ww.wp.xz.cn/plugins/search.php?q=ContactUs

Please let me know how to get that thumbnail in the results. Thank you so much! JFA / ContactUs.com