Well, I use Image::Magick for that
Example:

my $thumb= new Image::Magick;
open(IMAGE, "</path/$photof");
$thumb->Read(file=>\*IMAGE);
close(IMAGE);
$thumb->Resize(geometry=>'120X90');
$thumb->Write("/path/thumb_$photof");
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Of course, what is missing in there for your usage is simply a:

my ($height,$width) = $thumb->Get('width','height);
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before the resize, and add there your checks.
(the last part is untested).

Michalis