how to get random media id from media gallery

You can use get_posts() function to get random post, without additional php manipulation.
Just change default orderby attribute to “rand” and set number of posts attribute equals to “1”.

$image = get_posts( 
    array(
        'orderby'       => 'rand', //random order
        'numberposts' => 1, // numberposts, not posts_per_page
        'post_type'      => 'attachment', 
        'post_mime_type' => 'image', 
        'post_status'    => 'inherit' 

    ) 
);

//for testing purposes
echo $image[0]->ID;