Looking at the Wiki page for jpeg I see that the size marker tag is 0xFF 0xCO for baseline images and 0xFF 0xC2 for progressive images. Also marker for a JFIF is 0xFF 0xD8 0xFF Updating complete sub image_size to ...

sub image_size {
    return unless $_[0];
    my ($width, $height, $sig);
    if ( $_[0] =~ m/^GIF8..(....)/s ) {
        $sig = 'GIF';        # found GIF signature
        ($width, $height) = unpack( "SS", $1 );
    }
    elsif ( $_[0] =~ m/^^\xFF\xD8\xFF.{3}JFIF/s  ) {
        $sig = 'JPEG';
        ($height,$width) = unpack( "nn", $1 )
               if $_[0] =~ /\xFF\xC0...(....)/s; #Baseline Image
        ($height,$width) = unpack( "nn", $1 ) 
               if $_[0] =~ /\xFF\xC2...(....)/s; #Pogressive Images
    }
    elsif ( $_[0] =~ /^\x89PNG\x0d\x0a\x1a\x0a/  ) {
      $sig = 'PNG';        # found PNG signature     
    ($width, $height) = unpack( "NN", $1 ) if $_[0] =~ /IHDR(.{8})/s;
    }
    elsif ( $_[0] =~ /BM.{16}(.{8})/s ) {
        $sig = 'BMP';        # found bitmap sig
        ($width, $height) = unpack( "LL", $1);
    }
    return $width, $height, $sig;
}
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