For dropdown, checklist or optionlist attributes an icon can be added to the attribute values. On the Admin > Attributes page you will see the option to 'edit list values' which allows you to create, edit or remove possible values for these types of attributes. When doing so, you can upload an icon to the 'Icon File' box.

If the attribute is visible on the browse or search results pages then the icon will show next to the attribute value. If you do not see it immediately then you may need to reindex your assets.