How can I return multiple image attachments with wp_get_attachment_image_src

In PHP function can return it’s value only one time. After returning value, function dies.

If you want to return multiple values, you have to use array. So your code could look like this:

function get_images() {
    global $post;  // you don't use $post in your code, so it's redundant
    $size="medium";
    $attachments = get_children( array(
        'post_parent' => get_the_ID(),
        'post_status' => null,  // ??
        'numberposts'    => -1,  // you should use posts_per_page insted of numberposts
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'order' => 'ASC',
        'orderby' => 'menu_order'
    ) );
    if (empty($attachments)) {
        return '';
    }

    $images = array();
    foreach ( $attachments as $id  => $attachment ) {
        $images[] = wp_get_attachment_image_src($attachment->ID, $size );
    }
    return $images;
}

PS. But it has nothing to do with WordPress…