You'll want to make sure in core/common/ScreenshotsData.class.php (or similar named file), all the
public function xxx()...
are changed to (note the word static)
public static function xxx()...
Then if that doesn't work, instead of this
<?php Screenshots::show_newest_screenshot(); ?>
Try this
<?php
$screenshot = new Screenshots();
$screenshot->show_newest_screenshot();
?>