Not sure about you mean Profile Photo or Photo Gallery, so...

Change Profile Photo to Random:

Go to /templates/tmpl_uni/scripts/index.php

And find what is bellow here inside function:
function PageCompTopPhotosContent( $sCaption )

if ( $_GET['Mode_p'] == 'rand' or
$_GET['Mode_p'] == 'last' or
$_GET['Mode_p'] == 'top' )
$mode = $_GET['Mode_p'];
else
$mode = 'last';


Chang $mode = 'last';
To $mode = 'rand';



Change Photo Gallery to Random:

Go see more to inc/sharing.inc.php

And find what is bellow here inside function:
function PageCompSharePhotosContent($sCaption, $iID = 0)

$mode = $_GET['shPhotosMode'];
if( $mode != 'rand' && $mode != 'top' && $mode != 'last')
$mode = 'last';


Chang $mode = 'last';
To $mode = 'rand';