Get total attachment image count of a wordpress site media from functions file.

Img_count.php

/*
*Add this code to your theme's function file or site-specific plugins
*/
function img_count(){
    $query_img_args = array(
        'post_type' => 'attachment',
        'post_mime_type' =>array(
                        'jpg|jpeg|jpe' => 'image/jpeg',
                        'gif' => 'image/gif',
                'png' => 'image/png',
                ),
        'post_status' => 'inherit',
        'posts_per_page' => -1,
        );
    $query_img = new WP_Query( $query_img_args );
    echo $query_img->post_count;
}
/*
* Now call this function where you need to show the img count
*/
echo img_count();