To add images use the “Featured image” for a menu item

if you do not have this options it’s either hidden (go to screen options at the top of the page to enable/show it) or your theme does not have thumbnails enabled (tell the author about it. not my fault) if the auther doesn’t want to or cannot do anything about it, you can also try to just put the following code at the bottom of your theme’s – or better still child theme’s – functions.php file (before the closing ?> if any )

add_theme_support('post-thumbnails');

To set thumbnail/image sizes, go to WP->Settings->Media