You could add image field in _prepareForm() method of your admin form:
$fieldset->addField( 'imagefieldname', 'image', array( 'name' => 'imagefieldname', 'label' => __('Image'), 'title' => __('Image') ) );

You could add image field in _prepareForm() method of your admin form:

$fieldset->addField(
    'imagefieldname',
    'image',
    array(
        'name' => 'imagefieldname',
        'label' => __('Image'),
        'title' => __('Image')
    )

  );