As an added precaution to keep the grid intact when the second image is bigger than the first image. I made sure both images were sized right, so you could leave the button size options out. BUT, try loading a big image for the second image, and notice what happens when you specify button size, and not. When you do specify, the new big image will be contained and partially shown. When you don't, the whole grid gets re-arranged to accomadate the entire image.
sub get_image2{
my $bunny = $top->Photo(-file => 'somebig.jpg');
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return $bunny;
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}