I know, I know -- posting a reponse to my own question is lame -- but I did find a solution that worked out just fine. I wouldn't call it a solution so much as a workaround, but here is what I did: Replaced setImage() with the following:

sub makeimages {
      my $w = shift;
      foreach (qw /uparrow downarrow none/) {
         $w->{$_}->delete if (defined($w->{$_}));
         $w->{$_} = $w->Compound;
         $w->{$_}->Line;
         $w->{$_}->Text(
                        -text => $w->cget('-text'),
                       );
         if ($_ eq 'none') {
            $w->{$_}->Space(-width => 15);
         } else {
            $w->{$_}->Bitmap(-bitmap => $_);
         }
      }
      $w->OnDestroy( sub {
                           foreach (qw /uparrow downarrow none/) {
                              $w->{$_}->delete if (defined($w->{$_}));
                           }
                         }
                   );
   }
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Then I just call makeimages() in columnInsert and configure the images using a configure(-image) on the button in question when I need to change it. I figured that three images in memory at once is way better than one at a time if having one at a time leads to a memory leak. Anyway, when doing it this way, there is no leakage, so it seems to me that the problem only occurs after repeated Compound image creations and deletions.