The parameter is imgtype and it takes 2 values: news and face. I'll let the images do the talking.
Here's what you get when your search term is "gates":
You can see good old Bill, whose image I was looking for, appears the first time at the 13th place, after various gates.
Here's what you get when you add &imgtype=face at the end of the URL:
Clearly we're getting somewhere here - with more people pictures than abstract gates.
Now, when you use &imgtype=news, you get this:
Now you get pictures of Robert Gates, the US Secretary of Defense, who's been in the spotlight lately.
Why not give it a try? Search Google Images