Coding Our Web

Depends on your theme. Usually it’s functions.php in /wp-contents/themes/[your-theme]/

Hi Raikit,
Had the same problem with a theme named uDesign. Seems the theme had explicitly set:

if ( function_exists( 'add_theme_support' ) ) {
    add_theme_support( 'post-thumbnails', array('post', 'page') );
}

After I replaced this with:

if ( function_exists( 'add_theme_support' ) ) {
    add_theme_support( 'post-thumbnails' );
}

everything worked fine!