Here is an updated version that will cycle thru any legal graphic file in the pwd, fading from one image to the next, slideshow fashion. It will read each file on every cycle thru the list, so you may want to store the pixbufs in a hash so they only are read once.

#!/usr/bin/perl
use strict;
use warnings;
use Gtk2 '-init';

# a list to get most of the standard graphic formats into a glob
my @exts = qw(wbmp wmf jpeg jpg bmp gif pcx png pnm pbm 
               pgm ppm tga targa xbm tiff tif xpm );
 
@exts = map { $_ = "*.$_" } @exts;   #map to setup the glob below
my @files = <@exts>; #non recursive
if(scalar @files == 0){ die "No photo files found\n" }
print "@files\n";


my $mw = Gtk2::Window->new;
$mw->signal_connect('destroy', sub { Gtk2->main_quit });

my $vp = Gtk2::Viewport->new(undef, undef);
$mw->add($vp);

# start with base image
my $alpha =  0;
my $next = 0;
my $pb_base = Gtk2::Gdk::Pixbuf->new_from_file_at_scale ($files[0], 60
+0,600,0);
my $pb_base_orig = $pb_base->copy();
#get next pixbuf
push (@files,shift(@files));  
my $pb_next = Gtk2::Gdk::Pixbuf->new_from_file_at_scale ($files[0], 60
+0,600,0);

my $image = Gtk2::Image->new_from_pixbuf ($pb_base);
$vp->add($image);
$mw->set_title("fade demo");
$mw->show_all();

my $id = Glib::Timeout->add (100, \&sequence);

Gtk2->main;

sub sequence{

   if( $next ){
      # set base to be the previous next
      $pb_base = $pb_next;
      $pb_base_orig = $pb_base->copy();
      $image->set_from_pixbuf ($pb_base);
      Gtk2->main_iteration while Gtk2->events_pending;
      
      #get next pixbuf
      push (@files,shift(@files));  
      $pb_next = Gtk2::Gdk::Pixbuf->new_from_file_at_scale ($files[0],
+ 600,600,0);
      $next = 0;
      $alpha = 0;

    }else{
    
    $alpha += 5;
    print "$alpha\n"; 
    if ($alpha == 255){ $next = 1; }  
    
   #$pixbuf1->composite ($pixbuf, $dest_x, $dest_y, $dest_width, $dest
+_height,
    #       $offset_x, $offset_y, $scale_x, $scale_y, $interp_type, $o
+verall_alpha)

    $pb_next->composite ($pb_base, 0, 0, 600, 600,
           0, 0, 1, 1, 'nearest', $alpha);  # 0 <= alpha <= 255
    
    $image->set_from_pixbuf ($pb_base);
}

    Gtk2->main_iteration while Gtk2->events_pending;
    # prevents accumulation of pixbuf changes
    $pb_base = $pb_base_orig->copy();
 return 1;
}
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