Visible nowhere else that I can tell...Except in admin panel under "zenpage" tab.
Strict Standards: Only variables should be passed by reference in /home/--/public_html/zp-core/plugins/zenpage/zenpage-admin-functions.php on line 1186

(A few times)

I know, already reported on http://www.zenphoto.org/support/topic.php?id=4496#post-26553. Please try the Zenpage svn.