Build custom html to show posts thumbnail

Hi everyone. I installed and activated wordpress popular posts plugin. Also I am using this plugin as widget in my sidebar. I want to change the custom html of the output form. Following filters section I tried to use wpp_custom_html to build my custom html. But I dont know how to show the thumbnail of related post. For example: I can get title by using $popular->title. How can i get the featured image of the post?