Load url image full size in thumbnail, medium, large size

According to your description, in your template, it currently fetch image with specific size such as ‘thumbnail’ or ‘medium’ and so on. You want to change that with a main control and force them to use original image, right?

If so, you may try the filter post_thumbnail_size

add_filter( 'post_thumbnail_size', 'q363844_modify_image_thumbnail_size', 10, 2 );
function q363844_modify_image_thumbnail_size( $size, $post_id ) {
    // because you want to return original, you can set it to 'large'
    return 'large';
}

But in the long run, I think it is not good for your website traffic and optimisation.
You may consider to use a plugin by searching keywords regenerate thumbnail in plugin repository.