Even better, shows an image field to select an image
add_action('acf/render_field_settings/type=image', 'add_default_value_to_image_field', 20);
function add_default_value_to_image_field($field) {
acf_render_field_setting( $field, array(
'label' => __('Default Image ID','acf'),
'instructions' => __('Appears when creating a new post','acf'),
'type' => 'image',
'name' => 'default_value',
));
}