It's pretty hard to say what your exact problem is without seeing a minimal working code example, but using the ESP module, I think what you are looking for is a single usuable thread, that processes the images(looping thru them), then use an Idle->add in the thread after each thumbnail is created, to add it to the widget in the main Gtk2 thread.

Here is the basic idea of using Idle->add, it is the preferred way for threads to manipulate widgets in the main thread. The 2 second wait, you mention, hints at the eventloop being lazy in updating everything. The Idle->add will instruct the eventloop to add the new thumbnail to the display widget, at the first possible opening in event processing.

Glib::Idle->add is the great advance that Gtk2 made over Tk, to allow threads to access the gui widgets safely. And DO NOT spawn a new thread to process each image, that will be very slow. Reuse a single (or maybe a couple) of threads.

#!/usr/bin/perl
use warnings;
use strict;
use threads;
use threads::shared;
use Glib qw/TRUE FALSE/;
use Gtk2 qw/-init -threads-init/;

Glib::Object->set_threadsafe (TRUE);

#setup shared hash
my %shash;
share(%shash); #will work for first level keys

$shash{'go'} = 0;
$shash{'work'} = '';
$shash{'die'} = 0;

my $num:shared = 0;


my $window = Gtk2::Window->new('toplevel');
$window ->signal_connect( 'destroy' => \&delete_event );
$window->set_border_width(10);
$window->set_size_request(300,300);

my $vbox = Gtk2::VBox->new( FALSE, 6 );
$window->add($vbox);
$vbox->set_border_width(2);

my $hbox= Gtk2::HBox->new( FALSE, 6 );
my $hbox1 = Gtk2::HBox->new( FALSE, 6 );
$vbox->pack_end($hbox,FALSE,FALSE,0);
$vbox->pack_end (Gtk2::HSeparator->new, FALSE, FALSE, 0);
$vbox->pack_end($hbox1,FALSE,FALSE,0);
$hbox->set_border_width(2);
$vbox->pack_end (Gtk2::HSeparator->new, FALSE, FALSE, 0);

my $ebutton = Gtk2::Button->new_from_stock('gtk-quit');
$hbox->pack_end( $ebutton, FALSE, FALSE, 0 );
$ebutton->signal_connect( clicked => \&delete_event );

my $pbar = Gtk2::ProgressBar->new();
$pbar->set_pulse_step(.1);
$hbox->pack_start($pbar,1,1,0);


my $label_w_markup = Gtk2::Label->new();
$label_w_markup->set_markup("<span foreground=\"yellow1\" 
  size=\"40000\">$num</span>");

$vbox->pack_end($label_w_markup,FALSE,FALSE,4);    

######################################################

my $tbutton = Gtk2::Button->new_with_label('Run Thread');
 $hbox1->pack_start($tbutton , 1, 1, 0 );
my $lconnect = $tbutton->signal_connect( clicked => sub{ launch() });
my $sconnect;

$window->show_all();
$pbar->hide;   #needs to be called after show_all

#create 1 sleeping thread passing it the label and pbar to control
my $thread = threads->new(\&work, $label_w_markup, $pbar);

Gtk2->main;
######################################
sub delete_event {
  $shash{'go'} = 0;
  $shash{'die'} = 1;
  $thread->join;
  Gtk2->main_quit;
  return FALSE;
}
#######################################
sub launch{
   $pbar->show;
   $tbutton->set_label('Stop Thread');
   $tbutton->signal_handler_block($lconnect);
   $sconnect = $tbutton->signal_connect( clicked => sub{ stop() });
   $shash{'go'} = 1;
}
##################################################
sub stop{
   print "stopped\n";
   $shash{'go'} = 0;
   $pbar->hide;
   $tbutton->set_label('Run Thread');
   $tbutton->signal_handler_block ($sconnect);
   $tbutton->signal_handler_unblock ($lconnect);
}
#########################################################
sub work{
    
    my ($label,$pbar) = @_;
    $|++; 
    while(1){
       if($shash{'die'} == 1){ return }; 
      
       if ( $shash{'go'} == 1 ){
        
         $num = 0;
         while(1){
                $num++;
                
                if($num > 30){last}
                     
                 Glib::Idle->add(
               sub{
                if($shash{'die'} == 1){ return };
                $label->set_markup("<span foreground=\"yellow1\" 
                    size=\"40000\">$num</span>");
                    $pbar->pulse;
            return FALSE;
           });
                
            select(undef,undef,undef, .1);

            if($shash{'go'} == 0){last}
            if($shash{'die'} == 1){ return }; 
           }
       
       #if reach this point, the thread has finished
          print "my results = $num\n";   

         Glib::Idle->add(
               sub{
                if($shash{'die'} == 1){ return };
                $label->set_markup("<span foreground=\"green\" 
                    size=\"60000\">$num</span>");
                    $pbar->hide;
                    return FALSE;
           });
       
           $shash{'go'} = 0; #turn off self before returning      
       }else
         { select(undef,undef,undef,.1) } #sleep time
    }
}
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