Thumbnails problem

Hello,
First of all, please excuse me for my bad English, this is not my natural language.
I’m here because I don’t know WordPress well, then need your help to fix a little problem that I can’t find out how to fix by my own.

<b>What I want :</b>
I want my pages to display thumbnail pictures. I want those pictures to have a size of 940*200 when displaying the page on a computer screen, and a smaller size (responsivity, that kind of stuff) on phones.
I also want those pictures to have a size of 300*175 when I display a list of the pages (thumbnail+title).

<b>What I did :</b>
I first went in “settings>medias” to change the sizes. But whatever the size I put in there, it doesn’t change anything of the thumbnails that are displayed on my pages.
As I had no idea why it doesn’t work and what I’m supposed to do, I went to Google to find another way. I found some PHP codes.
I added this one to functions.php:

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 200, 300, true ); // Miniatures de l'accueil
add_image_size( 'miniature-archives', 200, 9999 ); // Miniatures des archives

and added this to page.php

<?php if ( has_post_thumbnail() ) { // dans la boucle
the_post_thumbnail();
}?>

It doesn’t work neither. I asked someone that told me to use the “Post Thumbnail Editor” plugin. But… neither.

<b>The problem :</b>
Whatever the size I set, whatever the feature I use, here is what my thumbnail look like: http://www.iut-lens.univ-artois.fr/wordpress/wp-content/uploads/2014/05/MMI-10×150.jpg?7499c
The thumbnail’s adress indeed says it’s a 10×150. But according to “Post Thumbnail Editor”, it’s supposed to be 200*300

Why do I have this 10×150 size? How can I do to change its size?

Thanks for your help….