in my opinion is difficult to find it. i always thought there is no option at all (so i used an extension convert bing to google). i just found it, because i searched "google". this option inside the setting panel should be repositioned. change such setting should happen faster, without need to search a lot.
Plus would be cool if we can set for all profiles "search with google", instead of repeat same things again and again (or installing the extension).
PS: nothing agains bing, but google search in my opinion is just better. bing image search is cool.