Should be pretty easy actually if you want to just use email. Building a more complex system within Plogger would be a bit of coding.
You can put a mailto: link somewhere in the picture output HTML, like after the EXIF table for instance at line ~247 of gallery.php.
$output .= '<a href="mailto:?subject=Check out this picture!&body=' . $_SERVER['PHP_SELF'] . '">Send to a Friend</a>
You can fool around with that idea I guess, you could do some different stuff like adding a text box for the email address and passing that into the mailto: url. You will have to experiment to make it right for your site.