I am using this code which currently the user calls as: [myimg no="1"] , [myimg no="2"] etc..

function myimg_shortcode($atts){
$atts = shortcode_atts( array('no' => 1,), $atts );
   $no = is_numeric( $atts['no'] ) ? (int) $atts['no'] : 1;
   $no  -=1 ;
}

add_shortcode('myimg', 'myimg_shortcode');

I am looking to add another optional string attribute so that for example the user will have the option to specify [myimg no="3" id="test"] where test should become the value of the string variable $id in the code. The no attribute is always required, whereas id should be optional.