4
You can do it relativly easy:
place this inside the template where you want the random image to appear:
![<{$random_img_alt}>](<{$random_img}>)
Now go into the php file that calls this template file. You are going to create a function inside this file that will have it's result assigned to $random_img using :
$xoopsTpl->assign('random_img',$random_img);
Open the file that controls the random image block (located in folder xCgal/Blocks, Use that code inside your own function. You might have to change the var that gets returned, remeber that you want an image
This is just a general aproach on how to do it. You will now have to dig into the code to figure out precisly how to make it work.