alchymyth on "Featured Image"

I have tried both templates that come with WP

Murphy's Law at its best - by choice of the theme developers, those two themes are not showing the featured image in posts or index pages.

there are some forum topics dealing with adding the featured image into those themes.

add_theme_support( 'post-thumbnails' ); does not do anything to show the featured images; for that you will need to have the_post_thumbnail() and related functions in your theme's templates.

I use a premium template from yootheme

have you contacted the theme's author directly for support with your 'problem'?
does the theme's demo or description suggest that the theme is using featured images?