add_theme_support( 'post-thumbnails' );
only adds the ability to use Featured Images. To have them appear in your site, you’ll need to add code to at least one of your template files, your active theme’s functions.php
file, or a custom plugin.
// in The Loop
if( has_post_thumbnail() ) {
the_post_thumbnail();
}
// etc.
References
has_post_thumbnail()
the_post_thumbnail()
- Post Thumbnails (now called Featured Images)
- Templates
- Writing a Plugin