Look-alikes are generally done by blending celebrity photo assets together with illustrated scenes and body gestures. Unlike full photo-comps, the heads are often the only photo based element, and it's usually filtered and painted into so that it feels part of the frame.
The most important thing to acheive when casting celebrities in a storyboard is undeniable recognition. The reason is two-fold. If the idea is testing, there needs to be no doubt in the focus group's mind as to who it is supposed to be, and secondly, the celebrity will often see the storyboard prior to giving approval to go ahead.