Please install advanced custom fields. For example you want to add a custom header on every type of post, i.e. pages. posts, custom posts and etc. You can just do what is being displayed in the screenshot: custome fields Then, to call it inside the theme/plugin you can call it as: 
get_field('global_header_image');
To display it, you can use:
the_field('global_header_image');
 . See more in the documentationDownload it here. For custom fields to be added inside the customize menu under appearance, you can use Theme Customization API.

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