Is it possible to use the image suffix (defined in options dialog) to overwrite the extension of the file, e.g. "picture.jpg.html" in "picture.html"? Thank you in advance.

Regards,
Steffen

No, not at present.

See also:
http://www.zenphoto.org/support/topic.php?id=1536&replies=6#post-10504
http://www.zenphoto.org/trac/ticket/287