Directory.GetFiles Method (System.IO) | Microsoft Docs[
^]
With
Directory.GetFiles (string, string)
you get all the files in the specified directory matching the specified pattern, the return is a string array.
string[] Images = System.IO.Directory.GetFiles (Path, @"bg_default*");
Checking if one or more images in the array and picking a "random" one of the array, should be easy.
Then you might set that as background.