[Plugin: WordPress Popular Posts] Thumbnail problem using the_post_thumbnail() function

I’m using read made template from 3rd party developer

the do not use, “grab the first image in post” but they use “Post Thumbnail”, which we can add it on media panel as “Featured Images” start from WordPress 2.9

Info: http://codex.wordpress.org/Template_Tags/the_post_thumbnail

How can I integrate post thumbnail show images from every post.

Thank you.