Thanks roju for all your helps, including those chats you sent me.

For your update 2, It still does not work for me even with active desktop disabled.

For update 1, I looked at the source code for Win32::Wallpaper, and made up this to mimic it:

 
use strict;
use warnings;

use Win32::API;
my $SystemParametersInfo = new Win32::API(qw(user32 SystemParametersIn
+foA NNPN N)) || die $^E;
$SystemParametersInfo->Call( 20, 0, "c:\\perl582\\bin\\yahoo.bmp",1 ) 
+|| die $^E;
[download]

Unfortunately it still does not work. But yes, I agree with you, my source code was most likely alright. Something is wrong ;-)