Just about everything in Gtk2 is event-driven, so if you add an event-watcher to the $image, as it's being loaded, it emits an "expose event" when displayed.

sub load_image {
   ...
   .... 
   .... 
    $vp->add( $image );

# add event watcher
   $image->signal_connect(
      event => sub {
         my ( $item, $event ) = @_;
         warn "event " . $event->type . "\n";
         return 0;    #return 1 prevents window from closing

# return 0 lets the signal thru
      }
   );
  ..... 
  .....
}
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Now, it would seem that you could just wait until the expose event occurs before proceeding. What the best way to do that is probably a resettable flag $is_image_exposed, and go into a non-blocking delay until it is 1.

I havn't tested that, and there may be other ways. Read "perldoc Gtk2::Widget" and look for things like "$widget->has_screen" ( and others).

The docs for Gtk2 are pretty meager, and require alot of experimentation to get the right code.

But if you ask me, just set the delay slower. :-)