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:
Jouke Visser, Perl 'Adept'
# 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
# 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);