Changing the detection on a cocos2d sprite

I have a sprite using spriteWithFile.

I have collision detection on this sprite which currently only detects开发者_运维问答 on the middle of the image. The middle of the image is currently detecting collision, so half half the image will be over the collision point when the detection is made.

How do I change this detection point on the character sprite?

Thanks

---

What about using some physics engine? Box2D for example. It's coming with cocos2d, it's fast, it's easy to use.