[Javascript] Javascript to change include filename

[Del Wegener]

Good Day;

One of my websites currently contains the following div

      <div id="blendme">
           <!--#include file="pic3.txt"-->
      </div>
My client wants to have 12 - 20 different pic_ordernn files.  A random 
number generator should be use decide which file to include (pic1.txt, 
pic2.txt, pic3.txt, ..., or pic15.txt).

I can write code to randomly create/select one of the filenames but how do I 
insert that filename in the above include statement?

It seems to me the include is serverside and my JS is client side.

Can that filename be a variable whose value is determined when the page is 
loaded?

Is this a PHP job?

Thanks for all your suggestions.

Del