So, what I did was edit the BMP in photoshop, editing the black border out to "transparent" and saved the file. This seems to work. I also created a FolderClosed.Bmp and adjusted the $tv->InsertItem calls so that you get Open Folders when it's selected and Closed Folders when unselected.

The code with just a couple changes is below along with links to the updated image files.

Good luck, I hope this helps.

Update: Per bbfu's reply, I was wrong about the transparency being in the new image. I was fooled by how photoshop treated the file and then saved it as a BMP, Sorry. I guess it was too early in the AM for me. I am looking at it further now.

Update: Ok, it's fixed, see latest files and code below: Details, Enjoy!

FolderOpen
OpenMask
FolderClosed
ClosedMask
Tree.pl

#!perl/bin/perl
use warnings;
use strict;

use Win32::GUI;

my $main = Win32::GUI::Window->new(
  -name => 'Main',
  -width => 300,
  -height => 250,
);

# NOTE: To use bitmap masks, the flag ILC_MASK (0x0001) must be set.
#  Also, using a 16 color bmp seem to require ILC_COLOR16 (0x0010)
my $il = Win32::GUI::ImageList->new(16, 16, 0x0011, 4, 5);

# This shows the image, but it's not masked
$il->Add('FolderOpen.bmp', 'FolderOpenMask.bmp');
$il->Add('FolderClosed.bmp', 'FolderClosedMask.bmp');

# As does this
#$il->AddBitmap(
#  Win32::GUI::Bitmap->new('FolderOpen.bmp', 0),
#  Win32::GUI::Bitmap->new('FolderOpenMask.bmp', 0),
#);

# And icon files don't work at all
#$il->AddBitmap(Win32::GUI::Bitmap->new('FolderOpen.ico', 1));

my $tv = $main->AddTreeView(
  -name => 'TreeView',
  -width => 280,
  -height => 200,
  -top => 10, -left => 5,
  -buttons => 1,
  -rootlines => 1,
  -imagelist => $il,
);

# Also, this doesn't seem to work, for some reason.
# The ImageList has to be passed when constructing the TreeView.
# Why?
#$tv->SetImageList($il, 24);

my $root = $tv->InsertItem(
  -text => 'Foo',
  -indent => 1,
  -selected => 0,
  -image => 1,
  -selectedimage => 0,
  );
$tv->InsertItem(
  -text => 'Bar',
  -indent => 1,
  -image => 1,
  -selectedimage => 0,
);
$tv->InsertItem(
  -text => 'Baz',
  -indent => 1,
  -image => 1,
  -selectedimage => 0,
  -parent => $root,
);

$main->Show(1);
Win32::GUI::Dialog();

sub Main_Terminate {
  return -1;
}
[download]

As far as I know, Windows bitmaps don't support a transparent "color". The images you linked to come up for me as having a white background. This looks fine if the user's settings have the TreeView's background color as white (which is the default) but if they change the "Window" setting in their Appearances section of Display properties, then the folder icons have a small white box around them.

This is why I was hoping to get "true" transparency using either a mask or an actual .ico file, which does support a transparent "color."

bbfu
Black flowers blossom
Fearless on my breath

Update: Ok, after wiping the egg off my face, I believe I have the answer now.

In order to use an image list with masks the flag "ILC_MASK" must be set in the call to ImageList_Create, Otherwise the image list contains only one bitmap. Also, To properly display your 16 color bitmaps I had to also include "ILC_COLOR16".

See the MS Doc here for more: ImageList_Create

Note: Win32::GUI::ImageList->new essentially just calls Win32::GUI::ImageList::Create(@_) in GUI.pm which maps to ImageList_Create(cx, cy, flags, cInitial, cGrow) in GUI.xs. So, to set the flags the MS Win32 API wants just supply them to the constructor.

# NOTE: To use bitmap masks, the flag ILC_MASK (0x0001) must be set.
#  Also, using a 16 color bmp seem to require ILC_COLOR16 (0x0010)
my $il = Win32::GUI::ImageList->new(16, 16, 0x0011, 4, 5);

# This Will NOW use masked BMP images
$il->Add('FolderOpen.bmp', 'FolderOpenMask.bmp');
$il->Add('FolderClosed.bmp', 'FolderClosedMask.bmp');
[download]

I also updated the files and links in my orignal reply: Re: Win32::GUI::TreeView and bitmap masks.

I hope this works better ;-)