Adding additional thumbnails to a page

I have this in the function.php

add_theme_support( 'post-thumbnails' );

And of course it gives me the option to add a “featured image” in the page admin panel.
I’d like to add a second thumbnail. so after searching the web I tried this:

add_theme_support( 'post-thumbnails' );
add_image_size( 'l3_thumbnail', 650, 150, true );

but of course that only gives me the same tnail but in a diff size.

how can i add the option of adding to a page & or post more than one featured image?
Thank you