How can I add a set featured image function to a theme that doesn’t already have it built in?

I’m a relative newbie to WP and PHP in general, but now that I’m into it, I’ve caught the “WP fever” and have ditched asp.net.

I finally found a quality theme that I like and I’m considering buying it; I have 4 blogs and counting, but I can’t get the basic version of the theme to display the set featured image link in my WP admin when creating a post or page. Vanilla install, WP 3.5.1.

The support forum isn’t that good, and nobody’s responded, but I’m quite sure they didn’t build this function into their theme.

Is it possible to add the set featured image function myself, even if I’m not a PHP junkie (yet)? Any guidance in this regard (whether I wanna hear it or not!) would be greatly appreciated!

You can add this line to your functions.php file to add the post thumbnail option to all post types:

add_theme_support( 'post-thumbnails' ); 

If you want it for only one post type, you pass an array of post type names as second argument:

add_theme_support( 'post-thumbnails', array( 'post' ) ); #shown in posts only

To display the thumbnail in your template files:

if ( has_post_thumbnail() ) {
    the_post_thumbnail();
} 

Read about Post Thumbails and add_theme_support().