OK, let me answer my own question so everyone here could benefit from it. This isn't documented in FAQ's or other places (like the POD-documentation of Tk). I found the answer in the Tk mailinglist archives.

For some fileformats (at least for PNG, GIF and JPEG), the data() method returns the data in base64 encoding. Also, the -data option expects the data to be base64 encoded.

To be complete, here's a snippet of code as example:

#!/usr/bin/perl -w
use strict;
use GD;
use Tk;
use Tk::JPEG;
use MIME::Base64;

# Create the main window
my $main = MainWindow->new();

# Open a JPG image in GD
my $im = GD::Image->new('feather.jpg');
# Create an empty image with the desired dimensions
my $resizedim = new GD::Image(120,120);
# Copy everything from $im and resize it into $resizedim
$resizedim->copyResized($im,0,0,0,0,120,120,$im->getBounds());
# encode the jpeg-output of the $resizedim
my $img = encode_base64($resizedim->jpeg());
# create a Photo object with the data and display it within a label
my $image = $main->Photo("button", -data => $img, -format => 'jpeg');
my $label= $main->Label(-image => "button");
$label->grid(-in => $main);

MainLoop;
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Jouke Visser, Perl 'Adept'