Since the release of WordPress 4.0 you can now add a thumbnail icon to your plugin, which is featured in the WordPress.org plugin repo.
Doing this is very similar to adding a banner to your plugin. Simply create a 128 x 128 sized image and save it as the following filename:
Then upload it to your plugin’s ‘Assets’ directory and commit it to your plugin repository via SVN. Once done, your new thumbnail will show up next to your plugin in the search.