It sounds like “Featured” or “Thumbnail” images would work. These are enabled by default for ‘posts’. For ‘pages’ they need to be enabled, but that is as simple as adding add_post_type_support( 'page', 'thumbnail' ); to your theme’s functions.php hooked on init, and, of course, modifying the theme template files to display the featured image.
function add_featured_to_post_wpse_83508() {
add_post_type_support( 'page', 'thumbnail' );
}
add_action('init','add_featured_to_post_wpse_83508');
The theme itself has to register its support with add_theme_support( 'post-thumbnails' );
In the Loop, the_post_thumbnail() will display the image