Number of seconds to read an image, per Medium's algorithm: https://blog.medium.com/read-time-and-you-bc2048ab620c

reading-time.php

<?php
function image_reading_seconds( $count ) {
 
   // Every image start with 3 seconds
   $reading_seconds = $count * 3;

   // The first 9 images also add the cumulative 
   // amount of seconds calculated by 10 seconds 
   // minus the image count for each image.
   $reading_seconds += 9 >= $count
      ? 10 * $count - intval( ( $count**2 + $count ) / 2.0 )
      : 45;  // 9 + 8 + 7 ... 3 + 2 + 1
	
   return $reading_seconds;

}