Well switching bewtween multiple web pages can be tricky, because how do you know it was succesfully loaded before you take the screenshot? But here is a script that works on linux, the one caveat is that you must run the script from the same virtual desktop that your browser is on.

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;
use X11::WMCtrl;
use Data::Dumper;

my $wmctrl = X11::WMCtrl->new;
printf("window manager is %s\n", $wmctrl->get_window_manager->{name});

my @windows = $wmctrl->get_windows;

my $wid;  # x window id
my $host; # virtual desktop

foreach my $app(@windows){
         my $title = $app->{title};
          if( $title =~ /Mozilla/i){
          print "$title\n";
          print Dumper(\$app),"\n\n";
          
          $wid = $app->{id};
          $host = $app->{host};

         }
   }

$wmctrl->switch($host);
#must be on same virtual desktop as the browser

#works
my $blob = `import -window $wid jpg:`;   

#my $blob = `import -`;  #postscript produced
#print "$blob\n";
# now $blob is in memory and you can do what you
# want with it.

my $output = Image::Magick->new(magick=>'jpg');
$output->BlobToImage( $blob );
$output->Resize(geometry=>'160x120');
$output->Write( $0.'.jpg');

exit;
# png works too
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