You can define the available options using the choices argument as seen in the example below.
Example
new \Kirki\Field\Radio_Image(
[
'settings' => 'radio_image_setting',
'label' => esc_html__( 'Radio Image Control', 'kirki' ),
'section' => 'section_id',
'default' => 'left',
'priority' => 10,
'choices' => [
'left' => get_template_directory_uri() . '/assets/images/left.png',
'center' => get_template_directory_uri() . '/assets/images/center.png',
'right' => get_template_directory_uri() . '/assets/images/right.png',
],
]
);