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.