Grepping cpan I contemplate the following...

    if (formatPtr == NULL) {
        if ((formatObj != NULL) && !matched) {
            Tcl_AppendResult(interp, "image format \"", formatString,
                    "\" is not supported", (char *) NULL);
        } else {
            Tcl_AppendResult(interp, "couldn't recognize image data",
                    (char *) NULL);
        }
        return TCL_ERROR;
    }
[download]

..but no clues for me.

Tk and images is always hard to manage, at least for me: search for my posts about the matter to see how many doubts I have..

Then I always get back to my own code that runs; in this case tk-jepg-custom-rotator published here too.

I found that a temporary GD image is needed and for Tk part you need an extra pass: MIME::Base64 encoding when you feed -data to Tk::Photo

Here a working version of your code:

use strict;
use warnings;
use Tk;
use Tk::JPEG;
use GD;
use MIME::Base64;               # <----------
my $file = $ARGV[0];
my $mw  = MainWindow->new();
my $ww = $mw->screenwidth;
my $wh = $mw->screenheight;

my $imgd = GD::Image->newFromJpeg($file);
my ($iw, $ih) = $imgd->getBounds();

print "screen size $ww x $wh and image is $iw x $ih\n";

my $imtk;
if ($iw > $ww or $ih > $wh) {
    my $wratio = $ww / $iw;
    my $ratio  = $wh / $ih;
    $ratio = $wratio if $wratio < $ratio;
    
    # temporary GD image needed (dunno why)     
    my $fullscreen = GD::Image->new($ww,$wh);  # <---------------
    
    $fullscreen->copyResampled( 
    
                            $imgd ,     # source image
                            0,          # X center of the destination 
+image
                            0,          # Y center of the destination 
+image
                            0,          # X specify the upper left cor
+ner of a rectangle in the source image
                            0,          # Y specify the upper left cor
+ner of a rectangle in the source image
                            $ww,        # final width
                            $wh,        # final height
                            $iw,        # source width to copy
                            $ih,        # source height to copy
                            
                                
    );
    $imtk = $mw->Photo(-data => MIME::Base64::encode( $fullscreen->jpe
+g()) ); # <---------------
} else {
    $imtk = $mw->Photo(-file => $file,         -format => 'jpeg');
}

$mw->Label(-image => $imtk)->pack(-expand => 1, -fill => 'both');
$mw->update;
MainLoop;
[download]

L*

PS you can find interesting also my picwoodpecker where image are also rotated based on exiff tags

PPS

> I cannot find anything in the Tk or GD docs to indicate what I'm doing wrong.

In the Tk::Photo documentation I read:

> -data => string

> Specifies the contents of the image as a string. The string can contain base64 encoded data or binary data. The format of the string must be one of those for which there is an image file format handler that will accept string data. If both the -data and -file options are specified, the -file option takes precedence.