Use ACF image field as thumbnail input

Standard Wordpress thumbnail input may be replaced with an ACF image field which is far more flexible/customizable. In order to do that you just have to give it a name of _thumbnail_id. I'm not sure if it is possible with a visual editor (because I don't use it), but for sure you can do that using ACF PHP API. Here is the code which will generate a field for thumbnail image selection:

array(
  'key' => 'field_thumbnail_id',
  'name' => '_thumbnail_id',
  'type' => 'image',
  'label' => 'Custom thumbnail input',
  'instructions' => 'Some more instructions below the label',
  'return_format' => 'id',
),

Of course all other image field settings might be used here e.g. min_width, min_height, max_size etc. For more info please refer to the official image field documentation at https://www.advancedcustomfields.com/resources/register-fields-via-php/.