please go to ..\plugins\jaw-widgets\widgets\ and open file jaw_banner_widget.php. You should see this

    protected $options = array(
        0 => array('id' => 'custom_banner',
            'description' => 'Banner',
            'type' => 'select',
            'values' => array(
                array('name' => 'Custom Banner 1', 'value' => '1'),
                array('name' => 'Custom Banner 2', 'value' => '2'),
                array('name' => 'Custom Banner 3', 'value' => '3'),
                array('name' => 'Custom Banner 4', 'value' => '4'),
                array('name' => 'Custom Banner 5', 'value' => '5')
            ),
            'default' => 'custom_banner_1',
        ),
    );

Simply add new line(s) after

array('name' => 'Custom Banner 5', 'value' => '5')

Do not forget to add a comma (if you want to add another line). Something like this

    protected $options = array(
        0 => array('id' => 'custom_banner',
            'description' => 'Banner',
            'type' => 'select',
            'values' => array(
                array('name' => 'Custom Banner 1', 'value' => '1'),
                array('name' => 'Custom Banner 2', 'value' => '2'),
                array('name' => 'Custom Banner 3', 'value' => '3'),
                array('name' => 'Custom Banner 4', 'value' => '4'),
                array('name' => 'Custom Banner 5', 'value' => '5'),
                array('name' => 'Your Custom Banner', 'value' => '6')
            ),
            'default' => 'custom_banner_1',
        ),
    );