Bitmaps use xbm files. Here's an example using betty_boop.xbm:

#! /usr/local/bin/perl 

use strict;
use warnings;
use Tk;

my $mw = MainWindow->new();
my $b  = $mw->Bitmap(
         -file       => '/path/to/betty_boop.xbm',
         -foreground => 'black',
         -background => 'white', ); 
my $l  = $mw->Label(
         -image => $b,
         -background => 'gray')->pack;

MainLoop;
[download]

And here's betty_boop.xbm

#define betty_boop_width 20
#define betty_boop_height 20
static unsigned char betty_boop_bits[] = {
   0x20, 0x00, 0x00, 0xe0, 0xff, 0x00, 0xf8, 0xff, 0x01, 0xfc, 0xff, 0
+x03,
   0xfc, 0xff, 0x07, 0xfc, 0xff, 0x07, 0x7c, 0x43, 0x03, 0xbe, 0xc1, 0
+x03,
   0x9f, 0x40, 0x06, 0x3c, 0x63, 0x03, 0x98, 0xf7, 0x02, 0xb8, 0xb5, 0
+x03,
   0xb8, 0x94, 0x02, 0xb8, 0xe3, 0x01, 0x38, 0x08, 0x03, 0x38, 0x1c, 0
+x01,
   0xe0, 0xff, 0x00, 0xf0, 0x60, 0x00, 0x78, 0x40, 0x00, 0xb0, 0x58, 0
+x00};
[download]

Or use whatever xbm that you want.