Hi,
enabling the sitemap extension with zenphoto version 1.4.0.2 [6757] (Official Build) generates warnings at http://www.mywebsite.com/index.php?sitemap :

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 292

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 406

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 422

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 406

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 406

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 406

Warning: Invalid argument supplied for foreach() in /mywebsite/zp-core/zp-extensions/sitemap-extended.php on line 422
etc.
Enabling the google image option or clearing the cache doesn't solve the problem.

Any brilliant idea ?

Cheers.

Try disabling the multilingual sitemap. All these errors refer to that.

Thank you for your answer, Acrylian.
Well... how ?

I don't see any multilingual option in the sitemap plugin (I use zenphoto version 1.4.0.2 [6757]).

Do you mean that it can't work on a mutlilingual site ?

Ah, sorry,right, it is tied to the general multilingual setting. So yes, I guess currently it does not work for a multilingual site then. No idea right now. Please open a ticket.

I'd like to, but I can't login or even register with a new account on Trac...

Trac, e.g. site sometimes suffers from heavy traffic so it does sometimes not work. I suggest to retry later.

ok, done :
http://www.zenphoto.org/trac/ticket/1832

Cheers.