Image analysis to reveal modifications...

A really interesting piece from Wired Magazine: http://blog.wired.com/27bstroke6/2007/08/researchers-ana.html

Compositing is everything.

I learned to value layering effects when I was into computer animation in the mid-90's. The concepts of good compositing translate to fashion photography and many other art forms.

Complex things tend to look more special.

If it looks like it took a long time to do by hand, that's less common now than it used to be, so there is value assumed. Add considered detail to make things look expensive. Just don't over-do it.