I thought about an optimized random image placeholder tag. So I created a better one that must given the directory path only. Then the replacement function creates a pure HTML image tag based on the list of files found in this directory.
The pro of this new placeholder tag is the better performace - it is not neccessary anymore for phpwcms to read and deliver the image via the random_image.php file in /img. But you can use the old random image tags furthermore.