What is the correct way to implement a non-rectangular sprite in Cocos2d?

I am working on a jigsaw puzzle. And therefor do our sprites have some strange forms (Jigsaw puzzle bricks). As of now, we have implemented the "detect" this way:

- (void)selectSpriteForTouch:(CGPoint)touchLocation {  
    CCSprite * newSprite = nil;

    // Loop array of sprites
    for (CCSprite *sprite in movableSprites) {

        // Check if sprite is hit.
        // TODO: Swap if with something better.
        if (CGRectContainsPoint(sprite.boundingBox, touchLocation)) {
            newSprite = sprite;
            break;
        }
    }
    if (newSprite != selSprite) {
        // Move along, nothing to see here
        // Not the problem
    }
}

- (BOOL)ccTouchBegan:(UITouch *)touch withEvent:(UIEvent *)event {
    CGPoint touchLocation = [self convertTouchToNodeSpace:touch];
    [self selectSpriteForTouch:touchLocation];
    return TRUE;
}

I know that the problem is in the keyword "sprite.boundingBox". Is there a better way of implementing this, OR is it a limit when using sprites based on .png's?

If so, how should I proceed?

I'm new to iPhone and game development :D