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().