Add a image uploader in the posts page backend

For custom fields in the backend (like an extra image or extra text) in a post backend we are using Advanced custom fields. This is for us the most easy way to add extra variables to the backend.

They also have a function especially for images or a gallery.

In your theme you can use get_field('name_of_the_field') in the loop of the post. Depending how the field is configured it returns an array, string of an id.