Entonces hazlo a la inversa, prueba esto:
Código PHP:
<?php
$readdir = opendir("./");
$files = array();
while (false !== ($file = readdir($readdir)))
{
if ($file != "." && $file != ".." && $file != "index.html" && $file != "imagenes.txt" && $file != "Thumbs.db" && $file != "back" && $file != "p_img.php") {
$files[] = $file;
}
}
closedir($readdir);
///
$imagenes = explode( "~", "imagenes.txt" );
$todelete = array_diff( $files, $imagenes );
foreach( $todelete as $file ) {
unlink( $file );
}
?>
Saludos.