Have your login's have content of:
// MR -- trick to make random background for login
if ((file_exists($path)) && (!file_exists("./.norandomimage"))) {
try {
$dirs = glob("{$path}/*");
$count = count($dirs);
$selnum = rand(1, $count);
$selimg = $dirs[$selnum];
$bckgrnd = "\tbackground-image: url({$selimg});\n".
"\tbackground-size: cover;\n".
"\tbackground-attachment: fixed;";
} catch (exception $e) {
// no action -- that mean using 'default' bckgrnd
}
}