I just downloaded this extension and made a modification in the mod_random_image_plus.php file on line 17. I did this because I didn't know what path to write in the folder parameter of the module, the complete path (http://ww..) ??

Well, I changed:

$folder = JPath::clean( $params->get( 'folder', JPATH_BASE . DS . 'images') );

For:

$folder = JPATH_BASE . JPath::clean( $params->get( 'folder', DS .'images') );

This way, we only need to add a relative path (/images/directory) to the site url like we used to on the regular mod_random_image.
Just a suggestion, but the module works really great.
Thanks!