What kind of photos do you mean? I think I don't understand the question. My answer would be different depending on if you're talking about stock photography to dress up a site, as opposed to say displaying a professional photographer's portfolio. For a professional photographer specializing in re-touching I think it can be helpful to show some before and after images.

Whether or not visitors can tell if a photo has been retouched would really depend on the skill of the photoshop user, in my opinion. There's a real art to making sure lighting and shadows look right in an "enhanced" photo.