how to add post thumbnail image??

i activated a theme which only shows post title with no post thumbnail image :{ thats why i want to display post thumbnail uploaded by user beside every post title. (and its going to be a public blog)

for example browse this website http://techtunes.com.bd/ though this site isn’t in english but you can easily see there’s a post thumbnail image situated left in every post.

and here is my site link http://www.sadhinotablog.co.cc

note: and there is no post thumbnail image adding system in my posting panel on dashboard.

———————need a easy solution——— thanks————-