Hmm.. not sure about the backgrounds/bitmaps for buttons. But here are the options..

# (@)METHOD:new Win32::GUI::Button(PARENT, %OPTIONS)
# Creates a new Button object;
# can also be called as PARENT->AddButton(%OPTIONS).
# Class specific %OPTIONS are:
# -align => left/center/right (default left)
# -valign => top/center/bottom
#
# -default => 0/1 (default 0)
# -ok => 0/1 (default 0)
# -cancel => 0/1 (default 0)

I would assume that generic options would work with this as well (ie -color etc...). But, don't take my word for it, I've never used win32::gui.

update: Played with it a little bit. Looks like what you need to do is something like this:

$TB = $W->AddToolbar(
    -left   =>  0,   
    -top    => 0, 
    -width  => $W->ScaleWidth-10, 
    -height => 100,
    -name   => "Toolbar",
);

$B = new Win32::GUI::Bitmap("unkfolder.bmp");
$C = new Win32::GUI::Bitmap("two.bmp");

$TB->SetBitmapSize(16, 16);

$TB->AddBitmap($B, 1);
$TB->AddBitmap($C, 2);

$TB->AddString("ONE");
$TB->AddString("TWO");

$TB->AddButtons(
    2,
    0, 1, 4, 0, 0,
    1, 2, 4, 0, 1,
);
[download]

That's pretty much stolen directly from the toolbar.pl sample. Basically, open the bitmaps, assign the size the bitmap will be on the toolbar, and then add the bitmaps to the relative button location (1,2,3 etc). Does that help?
Another update:
In the code above, ($B, 1) and ($C, 2) should read as ($B,0) and ($C,1).