when does art become voyeurism?

Panayiotis Lamprou's image, Portrait of My British Wife, on the shortlist of this year's Taylor Wessing photographic portrait prize. It is, as the Guardian's arts correspondent, Mark Brown, put it (perhaps understating the case somewhat), "arresting because of its intimacy". It begs the vexed question, when does art become voyeurism or, indeed, pornography?

Panayiotis Lamprou: the casual power of an intimate portrait