shortcode to output multiple images urls from media library id

Try this

function image_thingy($atts) {
    // Merge attribtes from shortcode with defaults
    extract(shortcode_atts(array(
        'id' => 1,
        ), $atts));

    // Extract id's from shortcode attributes and convert into an array
    $ids = explode(',',$atts['id']);

    $output=""; // Variable that holds the shortcode output, at the end this will be returned

    // Loop through ids and fetch urls, and add a comma with a blank space
    foreach($ids as $id){
       $output .= wp_get_attachment_url($id). ', ';
    }
    // remove comma and blank space from the end of $output, and finally return $output
    return rtrim($output,', ');
}
add_shortcode('imagez', 'image_thingy');

The following code wraps the urls in <img> tags:

function image_thingy($atts) {
    extract(shortcode_atts(array(
        'id' => 1,
        ), $atts));
    $ids = explode(',',$atts['id']);
    $output="";
    foreach($ids as $id){
       $output .= '<img src="'. wp_get_attachment_url($id) . '"/> ';
    }
    return $output;
}
add_shortcode('imagez', 'image_thingy');